Remove uses DCEInst and call ADCE

The algorithm used in DCEInst to remove dead code is very slow.  It is
fine if you only want to remove a small number of instructions, but, if
you need to remove a large number of instructions, then the algorithm in
ADCE is much faster.

This PR removes the calls to DCEInst in the load-store removal passes
and adds a pass of ADCE afterwards.

A number of different iterations of the order of optimization, and I
believe this is the best I could find.

The results I have on 3 sets of shaders are:

Legalization:

Set 1: 5.39 -> 5.01
Set 2: 13.98 -> 8.38
Set 3: 98.00 -> 96.26

Performance passes:

Set 1: 6.90 -> 5.23
Set 2: 10.11 -> 6.62
Set 3: 253.69 -> 253.74

Size reduction passes:

Set 1: 7.16 -> 7.25
Set 2: 17.17 -> 16.81
Set 3: 112.06 -> 107.71

Note that the third set's compile time is large because of the large
number of basic blocks, not so much because of the number of
instructions.  That is why we don't see much gain there.