[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