[VE][NFC] Fix use-after-free in PVFMK expansion

There is custom expansion code for packed VFMK Pseudos in the VE
backend.  This code erased the Pseudo without telling
ExpandPostRAPseudos about it, causing the generic expansion function to
access the erased Pseudo.  This bug triggered in the
test/CodeGen/VE/VELIntrinsics/vfmk.ll test with asan-enabled builds.

Detected by:
sanitizer-x86_64-linux-fast
(https://lab.llvm.org/buildbot/#/builders/5/builds/15393)