Add a hook to find out how the target handles shift amounts that are out of

Add a hook to find out how the target handles shift amounts that are out of
range.  Either they are undefined (the default), they mask the shift amount
to the size of the register (X86, Alpha, etc), or they extend the shift (PPC).

This defaults to undefined, which is conservatively correct.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@19677 91177308-0d34-0410-b5e6-96231b3b80d8