[InstCombine] Fix infinite loop due to bitcast <-> phi transforms

Fix for https://bugs.llvm.org/show_bug.cgi?id=44245.

The optimizeBitCastFromPhi() and FoldPHIArgOpIntoPHI() end up
fighting against each other, because optimizeBitCastFromPhi()
assumes that bitcasts of loads will get folded. This doesn't happen
here, because a dangling phi node prevents the one-use fold in
https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp#L620-L628 from triggering.

This patch fixes the issue by adding manually removing the old phis.

Differential Revision: https://reviews.llvm.org/D71164