Math dialect operations currently do not limit transformations

applied to them, which means that they potentially behave like

clang's -ffast-math mathematics. Clang marks math functions with

readnone attribute enabling more optimizations.

This change does the same for functions used by MathToLibm convertor.

In particular, this enables LLVM LICM for tan() call in

Polyhedron/mp_prop_design_11 compiled with flang.