R600/SI: Legalize operands after changing dst reg in FixSGPRCopies

Otherwise we may not legalize some illegal REG_SEQUENCE instructions.
R600/SI: Teach SIInstrInfo::moveToVALU() how to handle PHI instructions
R600/SI: Change default register class for i32 to SReg_32

SIFixSGPRCopies is smart enough to handle this now.
R600/SI: Stop using i128 as the resource descriptor type

Having i128 as a legal type complicates the legalization phase.  v4i32
is already a legal type, so we will use that instead.

This fixes several piglit tests.
R600: Add comment clariying use of sext for result of MUL_U24