X86: Better diagnostics for 32-bit vs. 64-bit mode mismatches.

When an instruction as written requires 32-bit mode and we're assembling
in 64-bit mode, or vice-versa, issue a more specific diagnostic about
what's wrong.

rdar://12700702

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167937 91177308-0d34-0410-b5e6-96231b3b80d8