[x86] add tests for rotate left/right with masked shifter; NFC

As noted in the test comment, instcombine now produces the masked
shift value even when it's not included in the source, so we should
handle this.

Although the AMD/Intel docs don't say it explicitly, over-rotating
the narrow ops produces the same results. An existence proof that
this works as expected on all x86 comes from gcc 4.9 or later:
https://godbolt.org/g/K6rc1A

llvm-svn: 310770