Avoid more invalid GIMPLE with register bases

Avoid more invalid GIMPLE with register bases

The Ada frontend, for example with gnat.dg/inline2_pkg.adb, tends
to create VIEW_CONVERT expressions with aggregate type even of
non-aggregate entities.  In this case for example

  return <retval> = (BIT_FIELD_REF <VIEW_CONVERT_EXPR<struct inline2_pkg__ieee_short_real>(number), 16, 16> & 32640) != 32640;

currently gimplification and SSA rewrite turn this into

  _1 = BIT_FIELD_REF <VIEW_CONVERT_EXPR<struct inline2_pkg__ieee_short_real>(number_2(D));

which is two operations on a register.  While as seen with PR109652
we might not want to completely rule out register to aggregate type
VIEW_CONVERTs we definitely do not want to stack multiple ops here.

The solution is to make sure the gimplifier puts a non-register as
the base object.  For the above this will add

  number.1 = number;

and use number.1 in the compound reference.  Code generation is
unchanged, FRE optimizes this to BIT_FIELD_REF <number_2(D), ...>.
I think BIT_FIELD_REF <VIEW_CONVERT (x), ...> could be always
rewritten into BIT_FIELD_REF <x, ...>, but that's a separate thing.

	* gimplify.cc (gimplify_compound_lval): When there's a
	non-register type produced by one of the handled component
	operations make sure we get a non-register base.