[mlir] Support fast-math friendly constants for identity value

Add an option to the family of `getIdentity` helper functions so that it is
possible to produce fast-math friendly constants.

For instance, for maxf the identity value is `-inf`, however, if the related
operations are lowered with fast-math (`noinf` in particular), then the value
becomes `poison` and chances are the whole codegen is not going to do what we
want.

To avoid this problem, we add an option to `getIdentity` and friends that
specifies whether a finite value needs to be produced or not.

The patch is NFC for all the code but the lowering of `linalg::softmax`
because we know we lower that with fast-math down the line.

I didn't audit the rest of the code to check if it would make sense to set
this boolean in more places.

Note: It feels kind of wrong to have to know what the lowering may do, but
I don't know what the right (at least short-term) solution is. Long term,
we may want a special "neutral element" attribute for the respective ops. I
didn't think too much about the implications for that.

Differential Revision: https://reviews.llvm.org/D156471

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