[InstCombine] call SimplifyICmpInst with correct context

Summary:
Fixes PR23809. Without passing the context to SimplifyICmpInst, we would
use the assume to prove that the condition feeding the assume is
trivially true (see isValidAssumeForContext in ValueTracking.cpp),
causing the removal of the assume which may be useful for later
optimizations.

Test Plan: pr23800.ll

Reviewers: hfinkel, majnemer

Reviewed By: hfinkel

Subscribers: henryhu, llvm-commits, wengxt, broune, meheff, eliben

Differential Revision: http://reviews.llvm.org/D10695

llvm-svn: 240683