Prevent register coalescing in functions whith setjmp

Summary:
In the the given example, a stack slot pointer is merged
between a setjmp and longjmp. This pointer is spilled,
so it does not get correctly restored, addinga undefined
behaviour where it shouldn't.

Change-Id: I60ec010844f2a24ce01ceccf12eb5eba5ab94abb

Reviewers: eli.friedman, thanm, efriedma

Reviewed By: efriedma

Subscribers: MatzeB, qcolombet, tpr, rnk, efriedma, hiraditya, llvm-commits, chill

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77767