[GVN] Forward source from memcpy

[dianqk]

The following IR can be transformed from load i24, ptr %dest to load i24, ptr %src so that we can know %arg equals %arg2.

define noundef i1 @wombat(ptr align 4 %src) {
bb:
  %dest = alloca [3 x i8], align 4
  %arg = load i24, ptr %src, align 4
  call void @llvm.memcpy.p0.p0.i64(ptr noundef nonnull align 4 dereferenceable(3) %dest, ptr noundef nonnull align 4 dereferenceable(3) %src, i64 3, i1 false)
  %arg2 = load i24, ptr %dest, align 4
  %select = icmp eq i24 %arg, %arg2
  call void @snork(ptr nonnull %dest)
  ret i1 %select
}

declare void @snork(ptr)

declare void @llvm.memcpy.p0.p0.i64(ptr noalias writeonly captures(none), ptr noalias readonly captures(none), i64, i1 immarg)

[dianqk]

The description provides a trival example. The original IR we needt to handle is https://godbolt.org/z/8zq4jdWP3.