SelectionDAG: Lower some range metadata to AssertZext

If a range has a lower bound of 0, add an AssertZext from the
nearest floor power of two.

This allows operations with some workitem intrinsics with known
maximum ranges to use fast 24-bit multiplies.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@260109 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed