[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)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338072 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed