swift234

[Devirtualizer] remove a hack that disables devirtualizing via specialized
conformance.

Revert r19137 and keep the testing case. Commit a fix that combines
substitutions from the specialized protocol conformance and substitutions from
the ApplyInst; then uses the full substitution list in substGenericArgs.

I had a small SIL testing case that runs "sil-opt -devirtualize", but due to
rdar://18120894, the testing case failed to work.

rdar://17440222

Performance -----
Before:
Totals,54,114129,114129,114129,0,0 (O)
Totals,54,81957,81957,81957,0,0 (Ounchecked)
Totals,54,83157,83157,83157,0,0 (run 2 of Ounchecked)

After:
Totals,54,109847,109847,109847,0,0 (O)
Totals,54,80301,80301,80301,0,0 (Ounchecked)


Swift SVN r21442