NFC; refactor getFrameIndexReferenceFromSP

Summary:
... into getFrameIndexReferencePreferSP.  This change folds the
fail-then-retry logic into getFrameIndexReferencePreferSP.

There is a non-functional but behaviorial change in WinException --
earlier if `getFrameIndexReferenceFromSP` failed we'd trip an assert,
but now we'll silently use the (wrong) offset from the base pointer.  I
could not write the assert I'd like to write ("FrameReg ==
StackRegister", like I've done in X86FrameLowering) since there is no
easy way to get to the stack register from WinException (happy to be
proven wrong here).  One solution to this is to add a `bool
OnlyStackPointer` parameter to `getFrameIndexReferenceFromSP` that
asserts if it could not satisfy its promise of returning an offset from
a stack pointer, but that seems overkill.

Reviewers: rnk

Subscribers: sanjoy, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D21427

llvm-svn: 272938