[ConstantHoisting] Avoid hoisting constants in GEPs that index into a struct type.

[ConstantHoisting] Avoid hoisting constants in GEPs that index into a struct type.

Summary:
Indices for GEPs that index into a struct type should always be
constants. This added more checks in `collectConstantCandidates:` which make
sure constants for GEP pointer type are not hoisted.

This fixed Bug https://bugs.llvm.org/show_bug.cgi?id=33538

Reviewers: ributzka, rnk

Reviewed By: ributzka

Subscribers: efriedma, llvm-commits, srhines, javed.absar, pirama

Differential Revision: https://reviews.llvm.org/D34576

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306704 91177308-0d34-0410-b5e6-96231b3b80d8