R600/SI: Fix dependency calculation for DS writes instructions in SIInsertWaits

In DS write instructions, the address operand comes before the value
operand(s) which is reversed from every other instruction type.

The SIInsertWait assumed that the first use for each instruction
was the value, so for DS write it was protecting the address
operand with s_waitcnt instructions when it should have been
protecting the value operand.

llvm-svn: 225289