[IRBuilder] Add IsInBounds parameter to CreateGEP()

We commonly want to create either an inbounds or non-inbounds GEP
based on a boolean value, e.g. when preserving inbounds from
existing GEPs. Directly accept such a boolean in the API, rather
than requiring a ternary between CreateGEP and CreateInBoundsGEP.

This change is not entirely NFC, because we now preserve an
inbounds flag in a constant expression edge-case in InstCombine.