R600: Match 24-bit arithmetic patterns in a Target DAGCombine

Moving these patterns from TableGen files to PerformDAGCombine()
should allow us to generate better code by eliminating unnecessary
shifts and extensions earlier.

This also fixes a bug where the MAD pattern was calling
SimplifyDemandedBits with a 24-bit mask on the first operand
even when the full pattern wasn't being matched.  This occasionally
resulted in some instructions being incorrectly deleted from the
program.

v2:
  - Fix bug with 64-bit mul
R600/SI: Handle INSERT_SUBREG in SIFixSGPRCopies