R600: Support for indirect addressing v3

Only implemented for R600 so far.  SI is missing implementations of a
few callbacks used by the Indirect Addressing pass and needs code to
handle frame indices.

At the moment R600 only supports array sizes of 16 dwords or less.
Register packing of vector types is currently disabled, which means that a
vec4 is stored in T0_X, T1_X, T2_X, T3_X, rather than T0_XYZW. In order
to correctly pack registers in all cases, we will need to implement an
analysis pass for R600 that determines the correct vector width for each
array.

v2:
  - Add support for i8 zext load from stack.
  - Coding style fixes

v3:
  - Don't reserve registers for indirect addressing when it isn't
    being used.
  - Fix bug caused by LLVM limiting the number of SubRegIndex
    declarations.

XXX: Fixes