glsl/lower_instructions: fix regression in dldexp_to_arith

glsl/lower_instructions: fix regression in dldexp_to_arith

The commit b4e198f47f842 changed the offset and bits parameters of the
bitfield insert operation from scalars to vectors. However, the lowering
of ldexp on doubles operates on each vector component and emits scalar
code (since it has to deal with the lower and upper 32-bit chunks of
each double component), so it needs its bits and offset parameters to
be scalars.

Fixes fp64 regression (crash) in:
spec/arb_gpu_shader_fp64/execution/built-in-functions/fs-ldexp-dvec4.shader_test

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>