GlobalISel: prevent heap use-after-free when looking up VReg.
GlobalISel: prevent heap use-after-free when looking up VReg.

Translating the constant can create more VRegs, which can invalidate the
reference into the DenseMap. So we have to look up the value again after all
that's happened.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@292675 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 6955b2c2f5057301533fecd2c28a0155fa49a29b)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm