[X86] Form MUL_IMM for multiplies with 3/5/9 to encourage LEA formation over load folding.

Previously we only formed MUL_IMM when we split a constant. This blocked load folding on those cases. We should also form MUL_IMM for 3/5/9 to favor LEA over load folding.

Differential Revision: https://reviews.llvm.org/D46040

llvm-svn: 330850