[mlir][Arith] Add pass for emulating unsupported float ops (#1079)

To complement the bf16 expansion and truncation patterns added to
ExpandOps, define a pass that replaces, for any arithmetic operation
op,
%y = arith.op %v0, %v1, ... : T
with
%e0 = arith.expf %v0 : T to U
%e1 = arith.expf %v1 : T to U
...
%y.exp = arith.op %e0, %e1, ... : U
%y = arith.truncf %y.exp : U to T

This allows for "emulating" floating-point operations not supported on
a given target (such as bfloat operations or most arithmetic on 8-bit
floats) by extending those types to supported ones, performing the
arithmetic operation, and then truncating back to the original
type (which ensures appropriate rounding behavior).

The lowering of the extf and truncf ops introduced by this
transformation should be handled by subsequent passes.

Reviewed By: rsuderman

Differential Revision: https://reviews.llvm.org/D154539