[LoadCombine] Avoid analysing dead basic blocks

Summary:
Dead basic blocks may be forming a loop, for which SSA form is
fulfilled, but with a circular def-use chain. LoadCombine could
enter an infinite loop when analysing such dead code. This patch
solves the problem by simply avoiding to analyse all basic blocks
that aren't forward reachable, from function entry, in LoadCombine.

Fixes https://bugs.llvm.org/show_bug.cgi?id=27065

Reviewers: mehdi_amini, chandlerc, grosser, Bigcheese, davide

Reviewed By: davide

Subscribers: dberlin, zzheng, bjope, grandinj, Ka-Ka, materi, jholewinski, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D31032

llvm-svn: 300034