[CodeExtractor] Fix sinking of allocas with multiple bitcast uses (PR42451)

An alloca which can be sunk into the extraction region may have more
than one bitcast use. Move these uses along with the alloca to prevent
use-before-def.

Testing: check-llvm, stage2 build of clang

Fixes llvm.org/PR42451.

Differential Revision: https://reviews.llvm.org/D64463

llvm-svn: 365660