Reorder LLVM passes, running mem2reg earlier.

This gives a ~30% shader optimization time improvement on blender.
Tested by comparing the dumped LLVM modules.
Current ordering:
time ~/llvm-git/obj/Release-Asserts/bin/opt l.bc  -constprop -instcombine
-mem2reg -gvn  -simplifycfg
real    0m1.126s
user    0m1.108s
sys     0m0.012s

With this patch:
time ~/llvm-git/obj/Release-Asserts/bin/opt l.bc -mem2reg -constprop -instcombine   -gvn  -simplifycfg
real    0m0.885s
user    0m0.880s
sys     0m0.000s

The overall improvement in blender is ~15%.
Blender without the patch takes 1m13s:
edwin     5934 87.6 11.5 729440 458296 pts/5   SLl+ 17:35   1:13 blender

Blender with the patch takes 1m3s:
edwin     5726 94.2 11.2 716424 446168 pts/5   SLl+ 17:32   1:03 blender

It is still slow with the patch, but better (most of the optimization time is
taken up by GVN, see LLVM PR7023).

Signed-off-by: Török Edwin <[email protected]>
Signed-off-by: José Fonseca <[email protected]>

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