Mark some FP intrinsics as safe to speculatively execute

Floating point intrinsics in LLVM are generally not speculatively
executed, since most of them are defined to behave the same as libm
functions, which set errno.

However, the only error that can happen  when executing ceil, floor,
nearbyint, rint and round libm functions per POSIX.1-2001 is -ERANGE,
and that requires the maximum value of the exponent to be smaller
than  the number of mantissa bits, which is not the case with any of
the floating point types supported by LLVM.

The trunc and copysign functions never set errno per per POSIX.1-2001.

Differential Revision: http://reviews.llvm.org/D18643

llvm-svn: 265262