[CodeGen] Handle mixed-width ops in mixed-sign mul-with-overflow lowering

The special lowering for __builtin_mul_overflow introduced in r320902
fixed an ICE seen when passing mixed-sign operands to the builtin.

This patch extends the special lowering to cover mixed-width, mixed-sign
operands. In a few common scenarios, calls to muloti4 will no longer be
emitted.

This should address the latest comments in PR34920 and work around the
link failure seen in:

  https://bugzilla.redhat.com/show_bug.cgi?id=1657544

Testing:
- check-clang
- A/B output comparison with: https://gist.github.com/vedantk/3eb9c88f82e5c32f2e590555b4af5081

Differential Revision: https://reviews.llvm.org/D55843

llvm-svn: 349542