[NFC][CodeGen][X86][AArch64] More tests for 'bit field extract' w/ constants

It would be best to introduce ISD::BitFieldExtract,
because clearly more than one backend faces the same problem.
But for now let's solve this in the x86-specific DAG combine.

https://bugs.llvm.org/show_bug.cgi?id=38938

llvm-svn: 342880