[CodeGen] Remove constant-folding dead code. NFC.

This code was specific to vector operations with scalar operands:
all the opcodes in FoldValue (via FoldConstantArithmetic) can't
match those criteria.

Replace it with an assert if that ever changes: at that point,
we might need to add back a splat BUILD_VECTOR.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266100 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed