RISC-V: rv32/DF: Prevent 2 SImode loads using XTheadMemIdx

When enabling XTheadFmv/Zfa and XThead(F)MemIdx, we might end up
with the following insn (registers are examples, but of correct class):

(set (reg:DF a4)
     (mem:DF (plus:SI (mult:SI (reg:SI a0)
			       (const_int 8))
		      (reg:SI a5))))

This is a result of an attempt to load the DF register via two SI
register loads followed by XTheadFmv/Zfa instructions to move the
contents of the two SI registers into the DF register.

The two loads are generated in riscv_split_doubleword_move(),
where the second load adds an offset of 4 to load address.
While this works fine for RVI loads, this can't be handled
for XTheadMemIdx addresses.  Coming back to the example above,
we would end up with the following insn, which can't be simplified
or matched:

(set (reg:SI a4)
     (mem:SI (plus:SI (plus:SI (mult:SI (reg:SI a0)
					(const_int 8))
			       (reg:SI a5))
		      (const_int 4))))

This triggered an ICE in the past, which was resolved in b79cd204,
which also added the test xtheadfmemidx-medany.c, where the examples
are from.  The patch postponed the optimization insn_and_split pattern
for XThead(F)MemIdx, so that the situation could effectively be avoided.

Since we don't want to rely on these optimization pattern in the future,
we need a different solution.  Therefore, this patch restricts the
movdf_hardfloat_rv32 insn to not match for split-double-word-moves
with XThead(F)MemIdx operands.  This ensures we don't need to split
them up later.

When looking at the code generation of the test file, we can see that
we have less GP<->FP conversions, but cannot use the indexed loads.
The new sequence is identical to rv32gc_xtheadfmv (similar to rv32gc_zfa).

Old:
[...]
	lla     a5,.LANCHOR0
	th.flrd fa5,a5,a0,3
	fmv.x.w a4,fa5
	th.fmv.x.hw     a5,fa5
.L1:
	fmv.w.x fa0,a4
	th.fmv.hw.x     fa0,a5
	ret
[...]

New:
[...]
	lla     a5,.LANCHOR0
	slli    a4,a0,3
	add     a4,a4,a5
	lw      a5,4(a4)
	lw      a4,0(a4)
.L1:
	fmv.w.x fa0,a4
	th.fmv.hw.x     fa0,a5
	ret
[...]

This was tested (together with the patch that eliminates the
XTheadMemIdx optimization patterns) with SPEC CPU 2017 intrate
on QEMU (RV64/lp64d).

gcc/ChangeLog:

	* config/riscv/constraints.md (th_m_noi): New constraint.
	* config/riscv/riscv.md: Adjust movdf_hardfloat_rv32 for
	XTheadMemIdx.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadfmemidx-xtheadfmv-medany.c: Adjust.
	* gcc.target/riscv/xtheadfmemidx-zfa-medany.c: Likewise.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>