[VE] Change the way to lower select

Change to use VEISD::CMOV in combineSelect for better optimization.
Support VEISD::CMOV in combineTRUNCATE also to optimize trancate.
Merge functions to handle condition codes to VE.h.  And add basic
CMOV patterns to VEInstrInfo.td.  Update regression tests also.

Reviewed By: efocht

Differential Revision: https://reviews.llvm.org/D135878