R600/SI: Fix verifier error with pseudo store instructions.

Use i32 instead of specifying SReg_32. When this is
the pseudo INDIRECT_BASE_ADDR, this would give a bogus
verifier error.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207770 91177308-0d34-0410-b5e6-96231b3b80d8