Split pseudo-instruction expansion into a separate pass, to make it

Split pseudo-instruction expansion into a separate pass, to make it
easier to debug, and to avoid complications when the CFG changes
in the middle of the instruction selection process.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119382 91177308-0d34-0410-b5e6-96231b3b80d8