RISC-V: Rework branch costing model for if-conversion

The generic branch costing model for if-conversion assumes a fixed cost
of COSTS_N_INSNS (2) for a conditional branch, and that one half of that
cost comes from a preceding condition-set instruction, such as with
MODE_CC targets, and then the other half of that cost is for the actual
branch instruction.  This is hardcoded for `if_info.original_cost' in
`noce_find_if_block' and regardless of the cost set for branches via
BRANCH_COST.

Then `default_max_noce_ifcvt_seq_cost' instructs if-conversion to prefer
a branchless sequence as costly as high as triple the BRANCH_COST value
set.  This is apparently to make up for the inability to accurately
guess the branch penalty.

Consequently for the BRANCH_COST of 3 we commonly set for tuning,
if-conversion will consider branchless sequences costing 3 * 3 - 2 = 7
instruction units more than a corresponding branch sequence.  For the
BRANCH_COST of 4 such as with `sifive-7-series' tuning this is even
worse, at 3 * 4 - 2 = 10.  Effectively it means a branchless sequence
will always be chosen if available, even a very inefficient one.

Rework the branch costing model to better match our architecture,
observing in particular that we have no preparatory instructions for
branches so that the cost of a branch is naked BRANCH_COST plus any
extra overhead the processing of a branch's source RTX might incur.

Provide TARGET_INSN_COST and TARGET_MAX_NOCE_IFCVT_SEQ_COST handlers
than that return suitable cost based on BRANCH_COST.  The latter hook
usually returns a value that is lower than the cost of the corresponding
branched sequence.  This is because we don't really want to produce a
branchless sequence that is more expensive than the original branched
sequence.  If this turns out too conservative for some corner case, then
this choice might be revisited.

Then we don't want to fiddle with `noce_find_if_block' without a lot of
cross-target verification, so add TARGET_NOCE_CONVERSION_PROFITABLE_P
defined such that it subtracts the fixed COSTS_N_INSNS (2) cost from the
cost of the original branched sequence supplied and instead adds actual
branch cost calculated from the conditional branch instruction used.  It
is then further tweaked according to simple analysis of the replacement
branchless sequence produced so as to cancel the cost of an extraneous
zero extend operation produced by `noce_try_store_flag_mask' as observed
with gcc/testsuite/gcc.target/riscv/pr105314.c.

Tweak the testsuite accordingly and set `-mbranch-cost=' explicitly for
the relevant cases so that the expected if-conversion transformation is
made regardless of the default BRANCH_COST value of tuning in effect.
Some of these settings will be lowered later on as deficiencies in
branchless sequence generation have been fixed that lower their cost
calculated by if-conversion.

	gcc/
	* config/riscv/riscv.cc (riscv_insn_cost): New function.
	(riscv_max_noce_ifcvt_seq_cost): Likewise.
	(riscv_noce_conversion_profitable_p): Likewise.
	(TARGET_INSN_COST): New macro.
	(TARGET_MAX_NOCE_IFCVT_SEQ_COST): New macro.
	(TARGET_NOCE_CONVERSION_PROFITABLE_P): New macro.

	gcc/testsuite/
	* gcc.target/riscv/zicond-primitiveSemantics_compare_imm_return_imm_imm.c:
	Explicitly set the branch cost.
	* gcc.target/riscv/zicond-primitiveSemantics_compare_imm_return_imm_reg.c:
	Likewise.
	* gcc.target/riscv/zicond-primitiveSemantics_compare_imm_return_reg_reg.c:
	Likewise.
	* gcc.target/riscv/zicond-primitiveSemantics_compare_reg_return_imm_imm.c:
	Likewise.
	* gcc.target/riscv/zicond-primitiveSemantics_compare_reg_return_imm_reg.c:
	Likewise.
	* gcc.target/riscv/zicond-primitiveSemantics_compare_reg_return_reg_reg.c:
	Likewise.