[RISCV] Unnecessary move of undefined with subregister liveness enabled

[lukel97]

define void @repeat_shuffle(<2 x double> %v, ptr noalias %q) {
  %w = shufflevector <2 x double> %v, <2 x double> poison, <4 x i32> <i32 0, i32 1, i32 0, i32 1>
  store <4 x double> %w, ptr %q
  ret void
}

When compiled with llc -mattr=+v, there's an unnecessary vmv1r.v v13, v10 emitted, which copies an implicit def to v13.

repeat_shuffle:                         # @repeat_shuffle
	.cfi_startproc
# %bb.0:
	vmv2r.v	v12, v8
	vsetivli	zero, 4, e64, m2, ta, ma
	vmv1r.v	v13, v10
	vslideup.vi	v8, v12, 2
	vse64.v	v8, (a0)
	ret

Compiling with -riscv-enable-subreg-liveness=0 avoids this:

repeat_shuffle:                         # @repeat_shuffle
	.cfi_startproc
# %bb.0:
                                        # kill: def $v8 killed $v8 def $v8m2
	vsetivli	zero, 4, e64, m2, ta, ma
	vmv2r.v	v10, v8
	vslideup.vi	v10, v8, 2
	vse64.v	v10, (a0)
	ret

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

[topperc]

cc: @BeMg

[BeMg]

This patch fix it.

[kito-cheng]

Fixed via 1e86abc914bb