R600/SI: Remove promotion of instructions to e64 forms.

Instructions are now generally selected to the e64 forms originally,
and shrunk down later. Rename foldOperands to legalizeOperands,
since that's really most of what it tries to do.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217959 91177308-0d34-0410-b5e6-96231b3b80d8