R600: Remove alu-split.ll test

The purpose of this test was to check boundary conditions for the size
of an ALU clause.  This test is very sensitive to changes to the
optimizer or scheduler, because it requires an exact number of ALU
instructions in order to remain valid.  It's not good to have a test
this sensitive, because it is confusing to developers who implement
optimizations and then 'break' the test.

I'm not sure if there is a good way to test these limits using lit, but
if I can come up with replacement test that isn't as sensitive I'll add
it back to the tree.