[CodeGen] Ignore unnamed bitfields before handling vector fields

We processed unnamed bitfields after our logic for non-vector field
elements in records larger than 128 bits.  The vector logic would
determine that the bit-field disqualifies the record from occupying a
register despite the unnamed bit-field not participating in the record
size nor its alignment.

N.B. This behavior matches GCC and ICC.

llvm-svn: 278656