ARM: tell LLVM about zext properties of ldrexb/ldrexh
Implementing this via ComputeMaskedBits has two advantages:
+ It actually works. DAGISel doesn't deal with the chains properly
in the previous pattern-based solution, so they never trigger.
+ The information can be used in other DAG combines, as well as the
trivial "get rid of truncs". For example if the trunc is in a
different basic block.
rdar://problem/
16227836
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205540
91177308-0d34-0410-b5e6-
96231b3b80d8