Revert "Revert "[AArch64][GlobalISel] Optimize G_FCMP + G_SELECT pairs when G_SELECT is fp""

When looking through copies, make sure to not try to find the vreg def of a physreg.
Normally getVRegDef will return nullptr in this case, but if there happens to be
multiple defs then it will assert.

This fixes PR42129.

llvm-svn: 362666