add the ability for TargetData to return information about legal integer

add the ability for TargetData to return information about legal integer
datatypes on a given CPU.  This is intended to allow instcombine and other
transformations to avoid converting big sequences of operations to an
inconvenient width, and will help clean up after SRoA.  See also "Adding
legal integer sizes to TargetData" on Feb 1, 2009 on llvmdev, and PR3451.

Comments welcome.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86370 91177308-0d34-0410-b5e6-96231b3b80d8