Now that tailduplication does not bork SSA form, run mem2reg earlier in gccas.

Now that tailduplication does not bork SSA form, run mem2reg earlier in gccas.
This tremendously improves the code generated by the LLVM optimizer, primarily
by making the inliner more aggressive.  For example, it improves the stepanov
benchmark from 55.56 mega-additions/sec to 98.04 Ma/s.  It also improves the
oopack/iterator benchmark from 338.3MFLOPS/s to 1103.4MFLOPS/s.  Less noteworthy,
it improves oopack/matrix from 573 -> 641 MFLOPS/s.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11053 91177308-0d34-0410-b5e6-96231b3b80d8