[RISCV] Add test case for miscompile in gather -> strided load combine. NFC

This shows the issue in llvm#82430, but triggers it via the widening SEW combine
rather than a GEP that RISCVGatherScatterLowering doesn't detect.

Author: lukel97

Diff for: llvm/test/CodeGen/RISCV/rvv/fixed-vectors-masked-gather.ll

@@ -15086,5 +15086,52 @@ define <32 x i64> @mgather_strided_split(ptr %base) {
   ret <32 x i64> %x
 }
 
+; FIXME: This is a miscompile triggered by the mgather ->
+; riscv.masked.strided.load combine. In order for it to trigger we need either a
+; strided gather that RISCVGatherScatterLowering doesn't pick up, or a new
+; strided gather generated by the widening sew combine.
+define <4 x i32> @masked_gather_widen_sew_negative_stride(ptr %base) {
+; RV32V-LABEL: masked_gather_widen_sew_negative_stride:
+; RV32V:       # %bb.0:
+; RV32V-NEXT:    addi a0, a0, -128
+; RV32V-NEXT:    li a1, -128
+; RV32V-NEXT:    vsetivli zero, 2, e64, m1, ta, ma
+; RV32V-NEXT:    vlse64.v v8, (a0), a1
+; RV32V-NEXT:    ret
+;
+; RV64V-LABEL: masked_gather_widen_sew_negative_stride:
+; RV64V:       # %bb.0:
+; RV64V-NEXT:    addi a0, a0, -128
+; RV64V-NEXT:    li a1, -128
+; RV64V-NEXT:    vsetivli zero, 2, e64, m1, ta, ma
+; RV64V-NEXT:    vlse64.v v8, (a0), a1
+; RV64V-NEXT:    ret
+;
+; RV32ZVE32F-LABEL: masked_gather_widen_sew_negative_stride:
+; RV32ZVE32F:       # %bb.0:
+; RV32ZVE32F-NEXT:    lui a1, 16392
+; RV32ZVE32F-NEXT:    addi a1, a1, 1152
+; RV32ZVE32F-NEXT:    vsetivli zero, 4, e32, m1, ta, ma
+; RV32ZVE32F-NEXT:    vmv.s.x v9, a1
+; RV32ZVE32F-NEXT:    vluxei8.v v8, (a0), v9
+; RV32ZVE32F-NEXT:    ret
+;
+; RV64ZVE32F-LABEL: masked_gather_widen_sew_negative_stride:
+; RV64ZVE32F:       # %bb.0:
+; RV64ZVE32F-NEXT:    addi a1, a0, 128
+; RV64ZVE32F-NEXT:    lw a2, 132(a0)
+; RV64ZVE32F-NEXT:    lw a3, 0(a0)
+; RV64ZVE32F-NEXT:    lw a0, 4(a0)
+; RV64ZVE32F-NEXT:    vsetivli zero, 4, e32, m1, ta, ma
+; RV64ZVE32F-NEXT:    vlse32.v v8, (a1), zero
+; RV64ZVE32F-NEXT:    vslide1down.vx v8, v8, a2
+; RV64ZVE32F-NEXT:    vslide1down.vx v8, v8, a3
+; RV64ZVE32F-NEXT:    vslide1down.vx v8, v8, a0
+; RV64ZVE32F-NEXT:    ret
+  %ptrs = getelementptr i32, ptr %base, <4 x i64> <i64 32, i64 33, i64 0, i64 1>
+  %x = call <4 x i32> @llvm.masked.gather.v4i32.v32p0(<4 x ptr> %ptrs, i32 8, <4 x i1> shufflevector(<4 x i1> insertelement(<4 x i1> poison, i1 true, i32 0), <4 x i1> poison, <4 x i32> zeroinitializer), <4 x i32> poison)
+  ret <4 x i32> %x
+}
+
 ;; NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
 ; RV64: {{.*}}