[SelectionDAG]  Handle big endian target BITCAST in computeKnownBits()

The BITCAST handling in computeKnownBits() previously only worked for little
endian.

This patch reverses the iteration over elements for a big endian target which
allows this to work in this case also.

SystemZ test case.

Review: Eli Friedman
https://reviews.llvm.org/D44249

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327764 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed