[PATCH 7/14][Testsuite] Add tests of reductions using whole-vector-shifts (multiplication)

[Alan Lawrence]

For reduction operations (e.g. multiply) that don't have such a tree code ,or 
where the target platform doesn't define an optab handler for the tree code, we 
can perform the reduction using a series of log(N) shifts (where N = #elements 
in vector), using the VEC_RSHIFT_EXPR=whole-vector-shift tree code (if the 
platform handles the vec_shr_optab).

First stage is to add some tests of non-(min/max/plus) reductions; here, 
multiplies. The first is designed to be non-foldable, so we make sure the 
architectural instructions line up with what the tree codes specify. The second 
is designed to be easily constant-propagated, to test the (currently 
endianness-dependent) constant folding code.

In lib/target-supports.exp, I've defined a new 
check_effective_target_whole_vector_shift, which I intended to define to true 
for platforms with the vec_shr optab. However, I've not managed to make this 
test pass on PowerPC - even with -maltivec, -fdump-tree-vect-details gives me a 
message about the target not supporting vector multiplication - so I've omitted 
PowerPC from the whole_vector_shift. This doesn't feel right, suggestions 
welcomed from PowerPC maintainers?

Tests passing on arm-none-eabi and x86_64-none-linux-gnu;
also verified the scan-tree-dump part works on ia64-none-linux-gnu (by compiling 
to assembly only).
(Tests are not run on AArch64, because we have no vec_shr_optab at this point; 
PowerPC, as above; or MIPS, as check_effective_target_vect_int_mult yields 0.)

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_whole_vector_shift):
	New.

	* gcc.dg/vect/vect-reduc-mul_1.c: New test.
	* gcc.dg/vect/vect-reduc-mul_2.c: New test.
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