Add "SkipDead" parameter to TargetInstrInfo::DefinesPredicate

Some instructions may be removable through processes such as IfConversion,
however DefinesPredicate can not be made aware of when this should be considered.
This parameter allows DefinesPredicate to distinguish these removable instructions
on a per-call basis, allowing for more fine-grained control from processes like
ifConversion.

Renames DefinesPredicate to ClobbersPredicate, to better reflect it's purpose

Differential Revision: https://reviews.llvm.org/D88494