[InstCombine] Remove redundant evaluateGEPOffsetExpression() fold (NFCI)

If we go through the generic EmitGEPOffset code, the resulting
expression can be (and is) reduced in the same way this code did
manually. There are no changes in lit tests or llvm-test-suite.

This fold predates the time where we started adding nsw to the adds
created by EmitGEPOffset, so it was likely needed back then.

This might not actually be NFC due to worklist order changes etc.