[DAG] visitVSELECT - don't wait for truncation of sub before attempting to...

[DAG] visitVSELECT - don't wait for truncation of sub before attempting to match with getTruncatedUSUBSAT

Fixes some X86 PSUBUS regressions encountered in D127115 where the truncate was being replaced with a PACKSS/PACKUS before the fold got called again