[NewGVN] Create a StoreExpression instead of a VariableExpression.

[NewGVN] Create a StoreExpression instead of a VariableExpression.

In the case where we have an operand defined by a lod of the
same memory location. Historically this was a VariableExpression
because we wanted to make sure they ended up in the same class,
but if we create the right expression, they end up in the same
class anyway.

Fixes PR32897. Thanks to Dan for the detailed discussion and the
fix suggestion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303475 91177308-0d34-0410-b5e6-96231b3b80d8