swift234

Implement a new global dead store elimination (DSE) with iterative data flow. This DSE
should eventually replace old DSE in LoadStoreOpts.cpp.
Currently, the new global DSE is under the EnableGDSE flag which is set to false by default.
Subsequent patches will keep improving it and when it can handle all cases handled by the old DSE
for stdlib and PerfTestSuite code, we can remove the old DSE and turn on the new DSE by default.

Swift SVN r32215