[DSE] Remove noop stores in MSSA.

Adds a simple fast-path check for the pattern:
v = load ptr
store v to ptr

I took the tests from the bugzilla post, I can add more if needed (but I think these should be sufficent).

Refs: https://bugs.llvm.org/show_bug.cgi?id=45795

Differential Revision: https://reviews.llvm.org/D79391