[RISCV] Add test for vmerge combine that should be prevented

The fadd in these test cases is constrained and may set fflags differently
depending on the active elements (the nofpexcept flag isn't set on the node).
Therefore to preserve semantics we shouldn't change its mask.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D154979

llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-vops.ll

@@ -361,6 +361,36 @@ define <vscale x 2 x float> @vpmerge_fadd(<vscale x 2 x float> %passthru, <vscal
   ret <vscale x 2 x float> %b
 }
 
+; This shouldn't be folded because we need to preserve exceptions with
+; "fpexcept.strict" exception behaviour, and masking may hide them.
+define <vscale x 2 x float> @vpmerge_constrained_fadd(<vscale x 2 x float> %passthru, <vscale x 2 x float> %x, <vscale x 2 x float> %y, <vscale x 2 x i1> %m, i64 %vl) strictfp {
+; CHECK-LABEL: vpmerge_constrained_fadd:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli a1, zero, e32, m1, ta, ma
+; CHECK-NEXT:    vfadd.vv v9, v9, v10
+; CHECK-NEXT:    vsetvli zero, a0, e32, m1, tu, ma
+; CHECK-NEXT:    vmerge.vvm v8, v8, v9, v0
+; CHECK-NEXT:    ret
+  %a = call <vscale x 2 x float> @llvm.experimental.constrained.fadd(<vscale x 2 x float> %x, <vscale x 2 x float> %y, metadata !"round.dynamic", metadata !"fpexcept.strict") strictfp
+  %b = call <vscale x 2 x float> @llvm.riscv.vmerge.nxv2f32.nxv2f32(<vscale x 2 x float> %passthru, <vscale x 2 x float> %passthru, <vscale x 2 x float> %a, <vscale x 2 x i1> %m, i64 %vl)
+  ret <vscale x 2 x float> %b
+}
+declare <vscale x 2 x float> @llvm.experimental.constrained.fadd(<vscale x 2 x float>, <vscale x 2 x float>, metadata, metadata)
+declare <vscale x 2 x float> @llvm.riscv.vmerge.nxv2f32.nxv2f32(<vscale x 2 x float>, <vscale x 2 x float>, <vscale x 2 x float>, <vscale x 2 x i1>, i64)
+
+; FIXME: This shouldn't be folded because we need to preserve exceptions with
+; "fpexcept.strict" exception behaviour, and masking may hide them.
+define <vscale x 2 x float> @vpmerge_constrained_fadd_vlmax(<vscale x 2 x float> %passthru, <vscale x 2 x float> %x, <vscale x 2 x float> %y, <vscale x 2 x i1> %m) strictfp {
+; CHECK-LABEL: vpmerge_constrained_fadd_vlmax:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli a0, zero, e32, m1, tu, mu
+; CHECK-NEXT:    vfadd.vv v8, v9, v10, v0.t
+; CHECK-NEXT:    ret
+  %a = call <vscale x 2 x float> @llvm.experimental.constrained.fadd(<vscale x 2 x float> %x, <vscale x 2 x float> %y, metadata !"round.dynamic", metadata !"fpexcept.strict") strictfp
+  %b = call <vscale x 2 x float> @llvm.riscv.vmerge.nxv2f32.nxv2f32(<vscale x 2 x float> %passthru, <vscale x 2 x float> %passthru, <vscale x 2 x float> %a, <vscale x 2 x i1> %m, i64 -1)
+  ret <vscale x 2 x float> %b
+}
+
 ; Test conversion by fptosi.
 define <vscale x 2 x i16> @vpmerge_fptosi(<vscale x 2 x i16> %passthru, <vscale x 2 x float> %x, <vscale x 2 x i1> %m, i32 zeroext %vl) {
 ; CHECK-LABEL: vpmerge_fptosi: