This patch adds MXCSR as a reserved physical register and models its use
by releated instructions. It also adds flag "mayRaiseFPException" for them.
Following what SystemZ and other targets does, only the current rounding
modes and the IEEE exception masks are modeled. *Changes* of the MXCSR
due to exceptions are not modeled.
We tend to prefer to split tests by vector width rather than features. So we should have 128-bit test with sse, avx, and avx512 command lines. A 256-bit test with avx and avx512 command lines. And a 512-bit test with avx512 command line. This way we can make sure a given function is generated in a similar way for all isas.