[flang] Lower F08 shift intrinsics

Lower F08 shift (shiftl, shiftr, shifta) and combined shift (dshiftl, dshiftr)
intrinsics. The combined shift intrinsics are implemented using the
definitions of shiftl and shiftr as described by the standard.

For non-conformant arguments to the shift intrinsics, the implementation tries
to replicate the behavior of other compilers if most of the other behave
consistently.

Differential Revision: https://reviews.llvm.org/D129316