Fix PR3453 and probably a bunch of other potential

Fix PR3453 and probably a bunch of other potential
crashes or wrong code with codegen of large integers:
eliminate the legacy getIntegerVTBitMask and
getIntegerVTSignBit methods, which returned their
value as a uint64_t, so couldn't handle huge types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63494 91177308-0d34-0410-b5e6-96231b3b80d8