[X86] Remove GetLo8XForm and use GetLo32XForm instead. NFCI

We were using this to create an AND32ri8 node from a 64-bit and, but that node
normally still uses a 32-bit immediate. So we should just truncate the existing
immediate to i32. We already verified it has the same value in bits 31:7.

llvm-svn: 356868