[SCEV] Add an expandAddToGEP overload for a single operand. NFC.

Only wanting to pass a single SCEV operand to use as the offset of
the GEP is a common operation. Right now this requires creating a
temporary stack array at every call site. Add an overload
that encapsulates that pattern and simplify the call sites.

Suggested-By: sanjoy (in https://reviews.llvm.org/D49832)
llvm-svn: 338072