R600/SI: Lower BUILD_VECTOR to REG_SEQUENCE v2

Using REG_SEQUENCE for BUILD_VECTOR rather than a series of INSERT_SUBREG
instructions should make it easier for the register allocator to coalasce
unnecessary copies.

v2:
  - Use an SGPR register class if all the operands of BUILD_VECTOR are
    SGPRs.

llvm-svn: 188427