[InstSimplify] remove 'nsz' requirement for frem 0, X

[InstSimplify] remove 'nsz' requirement for frem 0, X

From the LangRef definition for frem:
"The value produced is the floating-point remainder of the two operands.
This is the same output as a libm ‘fmod‘ function, but without any
possibility of setting errno. The remainder has the same sign as the
dividend. This instruction is assumed to execute in the default
floating-point environment."

llvm-svn: 327626