[InstCombine] use m_APInt in foldICmpWithConstant; NFCI

There's some formatting and pointer deref ugliness here that I intend to fix in
subsequent patches. The overall goal is to refactor the obnoxiously long switch
and incrementally remove the restriction to scalar types (allow folds for vector
splats). This patch introduces the use of m_APInt which means the RHSV reference
is now a pointer (and may have matched a vector splat), but the check of 'RHS' 
remains, so vector folds are disallowed and no functional change is intended.

llvm-svn: 278816