[InstCombine] when calling conventions are compatible, don't convert the call to undef idiom

D24453 enabled libcalls simplication for ARM PCS. This may cause
caller/callee calling conventions mismatch in some situations such as
LTO. This patch makes instcombine aware that the compatible calling
conventions differences are benign (not emitting undef idom).

Differential Revision: https://reviews.llvm.org/D99773