[X86] Enable fp128 as a legal type with SSE1 rather than with MMX.

FP128 values are passed in xmm registers so should be asssociated
with an SSE feature rather than MMX which uses a different set
of registers.

llc enables sse1 and sse2 by default with x86_64. But does not
enable mmx. Clang enables all 3 features by default.

I've tried to add command lines to test with -sse
where possible, but any test that returns a value in an xmm
register fails with a fatal error with -sse since we have no
defined ABI for that scenario.

llvm-svn: 370682