vect: Fix wrong shift_n after widening on BE [PR107338]

vect: Fix wrong shift_n after widening on BE [PR107338]

As PR107338 shows, with the use of widening loads, the
container_type can become a wider type, it causes us to
get wrong shift_n since the BIT_FIELD_REF offset actually
becomes bigger on BE.  Taking the case in PR107338 as
example, at the beginning the container type is short and
BIT_FIELD_REF offset is 8 and size is 4, with unpacking to
wider type int, the high 16 bits are zero, by viewing it
as type int, its offset actually becomes to 24.  So the
shift_n should be 4 (32 - 24 - 4) instead of 20 (32 - 8
- 4).

I noticed that if we move shift_n calculation early
before the adjustments for widening loads (container type
change), it's based on all the stuffs of the original
container, the shfit_n calculated there is exactly what
we want, it can be independent of widening.  Besides, I
add prec adjustment together with the current adjustments
for widening loads, although prec's subsequent uses don't
require this change for now, since the container type gets
changed, we should keep the corresponding prec consistent.

	PR tree-optimization/107338

gcc/ChangeLog:

	* tree-vect-patterns.cc (vect_recog_bitfield_ref_pattern): Move
	shfit_n calculation before the adjustments for widening loads.