[flang] Handle special case for SHIFTA intrinsic

This patch update the lowering of the shifta intrinsic to match
the behvior of gfortran. When the SHIFT value is equal to the
integer bitwidth then we handle it differently.
This is due to the operation used in lowering (`mlir::arith::ShRSIOp`)
that lowers to `ashr`.

Before this patch we have the following results:

```
SHIFTA(  -1, 8) =  0
SHIFTA(  -2, 8) =  0
SHIFTA( -30, 8) =  0
SHIFTA( -31, 8) =  0
SHIFTA( -32, 8) =  0
SHIFTA( -33, 8) =  0
SHIFTA(-126, 8) =  0
SHIFTA(-127, 8) =  0
SHIFTA(-128, 8) =  0
```

While gfortran is giving this:

```
SHIFTA(  -1, 8) = -1
SHIFTA(  -2, 8) = -1
SHIFTA( -30, 8) = -1
SHIFTA( -31, 8) = -1
SHIFTA( -32, 8) = -1
SHIFTA( -33, 8) = -1
SHIFTA(-126, 8) = -1
SHIFTA(-127, 8) = -1
SHIFTA(-128, 8) = -1
```

With this patch flang and gfortran have the same behavior.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D133104