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.

llvm-svn: 86370