middle-end/104497 - gimplification of vector indexing

The following attempts to address gimplification of

   ... = VIEW_CONVERT_EXPR<int[4]>((i & 1) != 0 ? inv : src)[i];

which is problematic since gimplifying the base object
? inv : src produces a register temporary but GIMPLE does not
really support a register as a base for an ARRAY_REF (even
though that's not strictly validated it seems as can be seen
at -O0).  Interestingly the C++ frontend avoids this issue
by emitting the following GENERIC instead:

   ... = (i & 1) != 0 ? VIEW_CONVERT_EXPR<int[4]>(inv)[i] : VIEW_CONVERT_EXPR<int[4]>(src)[i];

The proposed patch below fixes things up when using an rvalue
as the base is OK by emitting a copy from a register base to a
non-register one.  The ?: as lvalue extension seems to be gone
for C, C++ again unwraps the COND_EXPR in that case.

2022-02-11  Richard Biener  <rguenther@suse.de>

	PR middle-end/104497
	* gimplify.cc (gimplify_compound_lval): Make sure the
	base is a non-register if needed and possible.

	* c-c++-common/torture/pr104497.c: New testcase.

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