[AMDGPU] Remove fix up operand from SI_ELSE

Remove immediate operand from SI_ELSE which indicates if EXEC has
been modified.  Instead always emit code that handles EXEC and
remove unnecessary instructions during pre-RA optimisation.

This facilitates passes (i.e. SIWholeQuadMode) adding exec mask
manipulation post control flow lowering, and pre control flow
lower passes do not need to be aware of SI_ELSE handling.

Reviewed By: nhaehnle

Differential Revision: https://reviews.llvm.org/D89644