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expansion of vector shifts...

On sparc a simple test like (from the PR tree-optimization/53410 testcase):

typedef int V __attribute__((vector_size (4 * sizeof (int))));
typedef unsigned int W __attribute__((vector_size (4 * sizeof (int))));

f10 (W *p, W *q)
  *p = *p < (((const W) { 1U, 1U, 1U, 1U }) << *q);

aborts in convert_move() because we're trying to move a TImode value
into a V2SImode one.  How does that happen?

On sparc the generic tree vector layer turns the above expression into
two V2SImode shifts.  The *q parts of each shift are represented as:

	(subreg:V2SI (reg:TI xxx) 0)
	(subreg:V2SI (reg:TI xxx) 8)

When we get down into expand_shift_1(), that SUBREG is stripped out by
the SHIFT_COUNT_TRUNCATED code, and that's how we end up in the crash
by the time we reach convert_move() (via expand_binop() -->
expand_binop_directly() --> convert_modes() --> convert_move()).

Perhaps we should elide the SUBREG stripping if the subreg has a
vector mode?

Actually, what seems to confuse this code is that we're passing
TImode values around for this vector that the target doesn't have
direct support for.  The SUBREG stripper explicitly checks for
INTEGRAL_MODE_P, and indeed TImode is integral.

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