Revert "[InstCombine] when calling conventions are compatible, don't convert...

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

This reverts commit f4d682d6.

This caused a significant compile-time regression:
https://llvm-compile-time-tracker.com/compare.php?from=4b7bad9eaea2233521a94f6b096aaa88dc584e23&to=f4d682d6ce6c5b3a41a0acf297507c82f5c21eef&stat=instructions

Possibly this is due to overeager parsing of target triples.