[NFCI] Replace getAllocatedType with tracked value type in RewriteStatepointsForGC (#177440)
The allocas in RewriteStatepointsForGC are created with
LiveValue->getType(), so we can use Def->getType() (which equals the
alloca's type) instead of querying getAllocatedType().
Changes:
- Load instructions now use Def->getType() directly
- The ToClobber vector now stores (Type*, AllocaInst*) pairs to track
the original value's type for creating null constants
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>