[X86] Fix bug in x86_intrcc with arg copy elision

Summary:
Use a custom calling convention handler for interrupts instead of fixing
up the locations in LowerMemArgument. This way, the offsets are correct
when constructed and we don't need to account for them in as many
places.

Depends on D56883

Replaces D56275

Reviewers: craig.topper, phil-opp

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D56944

llvm-svn: 354837