Rounding modes have recently been added for `arith` FP operations
(#188458). This commit adds rounding modes to `math.fma`, following the
same design as for `arith` FP operations.
If a rounding mode is present, the LLVM lowering produces
`llvm.intr.experimental.constrained.fma`.
In the absence of a rounding mode, the rounding behavior is deferred to
the target backend.
Assisted-by: claude-opus-4.7-thinking-high
109f2a2856