Introduce 'UseSSEx' to force SSE legacy encoding

- Add 'UseSSEx' to force SSE legacy insn not being selected when AVX is
  enabled.

  As the penalty of inter-mixing SSE and AVX instructions, we need
  prevent SSE legacy insn from being generated except explicitly
  specified through some intrinsics. For patterns supported by both
  SSE and AVX, so far, we force AVX insn will be tried first relying on
  AddedComplexity or position in td file. It's error-prone and
  introduces bugs accidentally.

  'UseSSEx' is disabled when AVX is turned on. For SSE insns inherited
  by AVX, we need this predicate to force VEX encoding or SSE legacy
  encoding only.

  For insns not inherited by AVX, we still use the previous predicates,
  i.e. 'HasSSEx'. So far, these insns fall into the following
  categories:
  * SSE insns with MMX operands
  * SSE insns with GPR/MEM operands only (xFENCE, PREFETCH, CLFLUSH,
    CRC, and etc.)
  * SSE4A insns.
  * MMX insns.
  * x87 insns added by SSE.

2 test cases are modified:

 - test/CodeGen/X86/fast-isel-x86-64.ll
   AVX code generation is different from SSE one. 'vcvtsi2sdq' cannot be
   selected by fast-isel due to complicated pattern and fast-isel
   fallback to materialize it from constant pool.

 - test/CodeGen/X86/widen_load-1.ll
   AVX code generation is different from SSE one after fixing SSE/AVX
   inter-mixing. Exec-domain fixing prefers 'vmovapd' instead of
   'vmovaps'.

llvm-svn: 162919