RISC-V: Also handle sign extension in branch costing

Complement commit c1e8cb3d ("RISC-V: Rework branch costing model for
if-conversion") and also handle extraneous sign extend operations that
are sometimes produced by `noce_try_cmove_arith' instead of zero extend
operations, making branch costing consistent.  It is unclear what the
condition is for the middle end to choose between the zero extend and
sign extend operation, but the test case included uses sign extension
with 64-bit targets, preventing if-conversion from triggering across all
the architectural variants.

There are further anomalies revealed by the test case, specifically the
exceedingly high branch cost of 6 required for the `-mmovcc' variant
despite that the final branchless sequence only uses 4 instructions, the
missed conversion at -O1 for 32-bit targets even though code is machine
word size agnostic, and the missed conversion at -Os and -Oz for 32-bit
Zicond targets even though the branchless sequence would be shorter than
the branched one.  These will have to be handled separately.

	gcc/
	* config/riscv/riscv.cc (riscv_noce_conversion_profitable_p):
	Also handle sign extension.

	gcc/testsuite/
	* gcc.target/riscv/cset-sext-sfb.c: New test.
	* gcc.target/riscv/cset-sext-thead.c: New test.
	* gcc.target/riscv/cset-sext-ventana.c: New test.
	* gcc.target/riscv/cset-sext-zicond.c: New test.
	* gcc.target/riscv/cset-sext.c: New test.