target/115254 - fix gcc.dg/vect/vect-gather-4.c dump scanning

target/115254 - fix gcc.dg/vect/vect-gather-4.c dump scanning

The dump scanning is supposed to check that we do not merge two
sligtly different gathers into one SLP node but since we now
SLP the store scanning for "ectorizing stmts using SLP" is no
longer good.  Instead the following makes us look for
"stmt 1 .* = .MASK" which would be how the second lane of an SLP
node looks like.  We have to handle both .MASK_GATHER_LOAD (for
targets with ifun mask gathers) and .MASK_LOAD (for ones without).

Tested on x86_64-linux with and without native gather and on GCN
where this now avoids a FAIL.

	PR target/115254
	* gcc.dg/vect/vect-gather-4.c: Adjust dump scan.