Fix `unsafe-fp-math` attribute emission.

The conditions for which Clang emits the `unsafe-fp-math` function
attribute has been modified as part of
`84a9ec2f`.
In the backend code generators `"unsafe-fp-math"="true"` enable floating
point contraction for the whole function.
The intent of the change in `84a9ec2f`
was to prevent backend code generators performing contractions when that
is not expected.
However the change is inaccurate and incomplete because it allows
`unsafe-fp-math` to be set also when only in-statement contraction is
allowed.

Consider the following example
```
float foo(float a, float b, float c) {
  float tmp = a * b;
  return tmp + c;
}
```
and compile it with the command line
```
clang -fno-math-errno -funsafe-math-optimizations -ffp-contract=on \
  -O2 -mavx512f -S -o -
```
The resulting assembly has a `vfmadd213ss` instruction which corresponds
to a fused multiply-add. From the user perspective there shouldn't be
any contraction because the multiplication and the addition are not in
the same statement.

The optimized IR is:
```
define float @test(float noundef %a, float noundef %b, float noundef %c) #0 {
  %mul = fmul reassoc nsz arcp afn float %b, %a
  %add = fadd reassoc nsz arcp afn float %mul, %c
  ret float %add
}

attributes #0 = {
  [...]
  "no-signed-zeros-fp-math"="true"
  "no-trapping-math"="true"
  [...]
  "unsafe-fp-math"="true"
}
```
The `"unsafe-fp-math"="true"` function attribute allows the backend code
generator to perform `(fadd (fmul a, b), c) -> (fmadd a, b, c)`.

In the current IR representation there is no way to determine the
statement boundaries from the original source code.
Because of this for in-statement only contraction the generated IR
doesn't have instructions with the `contract` fast-math flag and
`llvm.fmuladd` is being used to represent contractions opportunities
that occur within a single statement.
Therefore `"unsafe-fp-math"="true"` can only be emitted when contraction
across statements is allowed.

Moreover the change in `84a9ec2f` doesn't
take into account that the floating point math function attributes can
be refined during IR code generation of a function to handle the cases
where the floating point math options are modified within a compound
statement via pragmas (see `CGFPOptionsRAII`).
For consistency `unsafe-fp-math` needs to be disabled if the contraction
mode for any scope/operation is not `fast`.
Similarly for consistency reason the initialization of `UnsafeFPMath` of
in `TargetOptions` for the backend code generation should take into
account the contraction mode as well.

Reviewed By: zahiraam

Differential Revision: https://reviews.llvm.org/D136786