The automatic exec size inference can accidentally mess things up if
we're not careful.  For instance, if we have

add(4)    g38.2<4>D    g38.1<8,2,4>D    g38.2<8,2,4>D

then the destination register will end up having a width of 2 with a
horizontal stride of 4 and a vertical stride of 8.  The EU emit code
sees the width of 2 and decides that we really wanted an exec size of 2
which doesn't do what we wanted.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>