[InstCombine] Don't rewrite phi-of-bitcast when the phi has other users

Judging by the existing comments, this was the intention, but the
transform never actually checked if the existing phi's would be removed.
See https://bugs.llvm.org/show_bug.cgi?id=44242 for an example where
this causes much worse code generation on AMDGPU.

Differential Revision: https://reviews.llvm.org/D71209