TableGen: Fix register class lane masks being too conservative.

When calculating the lanemask of a register class we have to include the
masks of subregisters supported by any of the class members, not just
the ones supported by all class members.

This fixes problems when coalescing towards a subclass with additional
subregisters available.

The attached testcase works fine as is, but does crash if you enable
subregister liveness on x86 without this change applied.

llvm-svn: 232652