[DAGCombiner] Fold IEEE `fmul`/`fdiv` by Pow2 to `add`/`sub` of exp

Note: This is moving D154678 which previously implemented this in
InstCombine. Concerns where brought up that this was de-canonicalizing
and really targeting a codegen improvement, so placing in DAGCombiner.

This implements:

```
(fmul C, (uitofp Pow2))
    -> (bitcast_to_FP (add (bitcast_to_INT C), Log2(Pow2) << mantissa))
(fdiv C, (uitofp Pow2))
    -> (bitcast_to_FP (sub (bitcast_to_INT C), Log2(Pow2) << mantissa))
```
The motivation is mostly fdiv where 2^(-p) is a fairly common
expression.

The patch is intentionally conservative about the transform, only
doing so if we:
    1) have IEEE floats
    2) C is normal
    3) add/sub of max(Log2(Pow2)) stays in the min/max exponent
       bounds.

Alive2 can't realistically prove this, but did test float16/float32
cases (within the bounds of the above rules) exhaustively.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D154805

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