[IR] Use pretty printing for mask in is_fpclass (#207653)
The second argument of `llvm.is_fpclass` is a mask, where each bit
represents a floating-point class. Understanding the check that the call
makes in an IR dump requires additional effort. To simplify the
interpretation of IR dumps, a previously implemented pretty-printing
mechanism can be used:
https://github.com/llvm/llvm-project/commit/39e7712ac520ccfc43383b3e9d6ea8cf2958b8e3.
This commit implements this mechanism for printing the arguments of
`llvm.is_fpclass`.
This change also fixes the printing of argument names: if an argument
name is absent, it is simply not printed.
