[RISCV] Add tests showing incorrect BUILD_VECTOR lowering

[frasercrmck]

Summary

These tests both use vector constants misidentified as VID sequences.
Because the initial run of elements has a zero step, the elements are
skipped until such a step can be identified. The bug is that the skipped
elements are never validated, even though the computed step is
incompatible across the entire sequence.

A fix will follow in a subseqeuent patch.

[craig.topper]

LGTM

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-buildvec.ll

 ; CHECK-NEXT:    vrgather.vx v9, v8, a0
 ; CHECK-NEXT:    vmv.v.v v8, v9
 ; CHECK-NEXT:    ret
   %x = extractelement <8 x i16> %v, i64 %idx
   %ins = insertelement <8 x i16> poison, i16 %x, i32 0
   %splat = shufflevector <8 x i16> %ins, <8 x i16> poison, <8 x i32> zeroinitializer
   ret <8 x i16> %splat
 }
+
+; FIXME: This is not a vid sequence!
+define <4 x i8> @buildvec_not_vid_v4i8_1() {
+; CHECK-LABEL: buildvec_not_vid_v4i8_1:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetivli zero, 4, e8, mf4, ta, mu
+; CHECK-NEXT:    vid.v v8
+; CHECK-NEXT:    ret
+  ret <4 x i8> <i8 0, i8 0, i8 2, i8 3>
+}
+
+; FIXME: This is not a vid sequence!
+define <4 x i8> @buildvec_not_vid_v4i8_2() {
+; CHECK-LABEL: buildvec_not_vid_v4i8_2:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetivli zero, 4, e8, mf4, ta, mu
+; CHECK-NEXT:    vid.v v8
+; CHECK-NEXT:    vrsub.vi v8, v8, 3
+; CHECK-NEXT:    ret
+  ret <4 x i8> <i8 3, i8 3, i8 1, i8 0>
+}