[X86] Don't force Nearest-Even rounding for VCVTPS2PH, use MXCSR.

Officially, we don't acknowledge non-default configurations of MXCSR,
as getting there would require usage of the FENV_ACCESS pragma (at
least insofar as rounding mode is concerned).

We don't support the pragma, so we can assume that the default
rounding mode - round to nearest, ties to even - is always used.

However, it's inconsistent with the rest of the instruction set,
where MXCSR is always effective (unless otherwise specified).
Also, it's an unnecessary obstacle to the few brave souls that use
fenv.h with LLVM.

Avoid the hard-coded rounding mode for fp_to_f16; use MXCSR instead.

llvm-svn: 259448