[GVN] missed load elimination when multiple stores contribute to a single value

[Colton1skees]

Both GVN and NewGVN fail to perform load elimination in cases where multiple stores contribute to a single constant value.

; Function Attrs: nofree norecurse nosync nounwind
define i16 @SampleFunc() local_unnamed_addr #0 {
entry:
  %0 = load ptr, ptr @memory, align 8
  %foo = getelementptr inbounds i8, ptr %0, i64 0
  %foo1 = getelementptr inbounds i8, ptr %0, i64 1
  store i8 7, ptr %foo, align 1
  store i8 -53, ptr %foo1, align 1
  %load158490 = load i16, ptr %foo, align 1
  ret i16 %load158490
}

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