[InlineCost] GetElementPtr with constant operands

If the GEP instruction contanins only constants as its arguments,
then it should be recognized as a constant. For now, there was
also added a flag to turn off this simplification if it causes
any regressions ("disable-gep-const-evaluation") which is off
by default. Once I gather needed data of the effectiveness of
this simplification, the flag will be deleted.

Reviewers: apilipenko, davidxl, mtrofin

Reviewed By: mtrofin

Differential Revision: https://reviews.llvm.org/D81026