RegAlloc: try to fail more gracefully when out of registers

Summary:
The emitError path allows the program to continue, unlike report_fatal_error.
This is friendlier to use cases where LLVM is embedded in a larger program,
because the caller may be able to deal with the error somewhat gracefully.

Change the number of requested NOP bytes in the AArch64 and PowerPC
test cases to avoid triggering an unrelated assertion. The compilation
still fails, as verified by the test.

Change-Id: Iafb9ca341002a597b82e59ddc7a1f13c78758e3d

Reviewers: arsenm, MatzeB

Subscribers: qcolombet, nemanjai, wdng, javed.absar, kristof.beyls, kbarton, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61489

llvm-svn: 360786