[GVN] Fix PHITransAddr crash scanning cross-function cast users (#211169)
When PHI-translating a load address through a select of module-level
constant expressions, the folded side becomes a global constant whose
use-list spans multiple functions. Scanning that use-list for an
available cast could return a cast from another function, which then
made DominatorTree::dominates() query a block from a different function
and trip an assertion.
This patch restricts the search to casts in the current function,
matching the existing guards on the GEP and add paths.
Fixes #211034