[RISCV] Add test for aliasing miscompile fixed by #83017. NFC

Previously we incorrectly removed the scalar load store pair here assuming it
was dead, when it actually aliased with the memset.  This showed up as a
miscompile on SPEC CPU 2017 when compiling with -mrvv-vector-bits, and was only
triggered by the changes in #75531.  This was fixed in #83017, but this patch
adds a test case for this specific miscompile.

For reference, the incorrect codegen was:

	vsetvli	a1, zero, e8, m4, ta, ma
	vmv.v.i	v8, 0
	vs4r.v	v8, (a0)
	addi	a1, a0, 80
	vsetivli	zero, 16, e8, m1, ta, ma
	vmv.v.i	v8, 0
	vs1r.v	v8, (a1)
	addi	a0, a0, 64
	vs1r.v	v8, (a0)

Author: lukel97

llvm/test/CodeGen/RISCV/rvv/pr83017.ll

@@ -0,0 +1,50 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 4
+; RUN: llc < %s -mtriple=riscv64 -mattr=+v -riscv-v-vector-bits-max=128 -verify-machineinstrs | FileCheck %s
+
+; This showcases a miscompile that was fixed in #83107:
+; - The memset will be type-legalized to a 512 bit store + 2 x 128 bit stores.
+; - the load and store of q aliases the upper 128 bits store of p.
+; - The aliasing 128 bit store will be between the chain of the scalar
+;   load/store:
+;
+;   t54: ch = store<(store (s512) into %ir.p, align 1)> t0, ...
+;   t51: ch = store<(store (s128) into %ir.p + 64, align 1)> t0, ...
+;
+;   t44: i64,ch = load<(load (s32) from %ir.q), sext from i32> t0, ...
+;   t50: ch = store<(store (s128) into %ir.p + 80, align 1)> t44:1, ...
+;   t46: ch = store<(store (s32) into %ir.q), trunc to i32> t50, ...
+;
+; Previously, the scalar load/store was incorrectly combined away:
+;
+;   t54: ch = store<(store (s512) into %ir.p, align 1)> t0, ...
+;   t51: ch = store<(store (s128) into %ir.p + 64, align 1)> t0, ...
+;
+;   // MISSING
+;   t50: ch = store<(store (s128) into %ir.p + 80, align 1)> t44:1, ...
+;   // MISSING
+;
+; - We need to compile with an exact VLEN so that we select an ISD::STORE node
+;   which triggers the combine
+; - The miscompile doesn't happen if we use separate GEPs as we need the stores
+;   to share the same MachinePointerInfo
+define void @aliasing(ptr %p) {
+; CHECK-LABEL: aliasing:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    lw a1, 84(a0)
+; CHECK-NEXT:    addi a2, a0, 80
+; CHECK-NEXT:    vsetivli zero, 16, e8, m1, ta, ma
+; CHECK-NEXT:    vmv.v.i v8, 0
+; CHECK-NEXT:    vs1r.v v8, (a2)
+; CHECK-NEXT:    vsetvli a2, zero, e8, m4, ta, ma
+; CHECK-NEXT:    vmv.v.i v12, 0
+; CHECK-NEXT:    vs4r.v v12, (a0)
+; CHECK-NEXT:    addi a2, a0, 64
+; CHECK-NEXT:    vs1r.v v8, (a2)
+; CHECK-NEXT:    sw a1, 84(a0)
+; CHECK-NEXT:    ret
+  %q = getelementptr inbounds i8, ptr %p, i64 84
+  %tmp = load i32, ptr %q
+  tail call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 96, i1 false)
+  store i32 %tmp, ptr %q
+  ret void
+}