[RISCV] Add test case showing vmerge fold miscompile with tail policy

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D158160

(cherry picked from commit 6e532f9)

Author: lukel97

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

@@ -1065,3 +1065,15 @@ define <vscale x 2 x i32> @vmerge_larger_vl_poison_passthru(<vscale x 2 x i32> %
   ret <vscale x 2 x i32> %b
 }
 
+; FIXME: The vadd's new policy should be tail undisturbed since the false op of
+; the vmerge moves from the the body to the tail, and we need to preserve it.
+define <vscale x 2 x i32> @vmerge_larger_vl_false_becomes_tail(<vscale x 2 x i32> %false, <vscale x 2 x i32> %x, <vscale x 2 x i32> %y, <vscale x 2 x i1> %m) {
+; CHECK-LABEL: vmerge_larger_vl_false_becomes_tail:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetivli zero, 2, e32, m1, ta, mu
+; CHECK-NEXT:    vadd.vv v8, v9, v10, v0.t
+; CHECK-NEXT:    ret
+  %a = call <vscale x 2 x i32> @llvm.riscv.vadd.nxv2i32.nxv2i32(<vscale x 2 x i32> poison, <vscale x 2 x i32> %x, <vscale x 2 x i32> %y, i64 2)
+  %b = call <vscale x 2 x i32> @llvm.riscv.vmerge.nxv2i32.nxv2i32(<vscale x 2 x i32> poison, <vscale x 2 x i32> %false, <vscale x 2 x i32> %a, <vscale x 2 x i1> %m, i64 3)
+  ret <vscale x 2 x i32> %b
+}