[InstCombine] Extend stacksave/restore elimination

Previously, InstCombine detected a pair of llvm.stacksave/stackrestore
instructions that are adjacent modulo debug instructions in order to
eliminate the llvm.stackrestore. This precludes situations where
intervening instructions (e.g. loads) preclude the llvm.stacksave and
llvm.stackrestore from becoming adjacent. This commit extends the logic
and allows for eliminating the llvm.stackrestore when the range of
instructions between them does not include any alloca or side-effect
causing instructions.

Signed-off-by: Itay Bookstein <itay.bookstein@nextsilicon.com>

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D113105