[GVN] Missing store to load forwarding for memset.pattern

https://llvm.godbolt.org/z/GzehrjsxW
```llvm
define <8 x i32> @test_pattern(ptr %p) {
call void @llvm.experimental.memset.pattern.p0.i32.i64(ptr %p, i32 42, i64 8, i1 false)
%load = load <8 x i32>, ptr %p
ret <8 x i32> %load
}

define <8 x i32> @test_plain(ptr %p) {
call void @llvm.memset.pattern.p0.i64(ptr %p, i8 42, i64 32, i1 false)
%load = load <8 x i32>, ptr %p
ret <8 x i32> %load
}
```
The latter case forwards, the former does not.

Start with the linked LLVM Godbolt reproducer and compare the generated results for test_pattern and test_plain. Trace the store-to-load forwarding path for the two memset pattern intrinsics; done means the experimental memset.pattern case forwards the loaded vector consistently with the plain case.

Written by the indexing model from the issue text.