Relax assert when setting access functions with invariant base pointers

Summary:
Instead of forbidding such access functions completely, we verify that their
base pointer has been hoisted and only assert in case the base pointer was
not hoisted.

I was trying for a little while to get a test case that ensures the assert is
correctly fired in case of invariant load hoisting being disabled, but I could
not find a good way to do so, as llvm-lit immediately aborts if a command
yields a non-zero return value. As we do not generally test our asserts,
not having a test case here seems OK.

This resolves http://llvm.org/PR31494



Suggested-by: Michael Kruse <llvm@meinersbur.de>

Reviewers: efriedma, jdoerfert, Meinersbur, gareevroman, sebpop, zinob, huihuiz, pollydev

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D28798

llvm-svn: 292213