[CodeGen]: fix error message for "=r" asm constraint
Nico Weber reported that the following code:
char buf; asm("" : "=r" (buf));
yields the "impossible constraint in asm: can't store struct into a register"
error message, although |buf| is not a struct (see
Make the error message more generic and add a test for it.
Also make sure other tests in x86_64-PR42672.c check for the full error
Reviewers: eli.friedman, thakis
Differential Revision: https://reviews.llvm.org/D66948