[InstCombine] Handle select inst when eliminating constant memcpy

Allow iterating through SelectInst use of the alloca when
checking if it is only ever overwritten from constant memory.
Recursively determine if the SelectInst is replacable and insert
it into the Worklist if so. Finally, define a new SelectInst to
replace the old one, with both of it's values replaced according
to the WorkMap.

Differential Revision: https://reviews.llvm.org/D136524