[AMDGPU] Use new opcode for indexed vgpr reads

Introduce V_MOV_B32_indirect_read for indexed vgpr reads
(and rename the old V_MOV_B32_indirect to
V_MOV_B32_indirect_write) so they can be unambiguously
distinguished from regular V_MOV_B32_e32. Previously they
were distinguished by looking for extra implicit operands
but this is fragile because regular moves sometimes have
extra implicit operands too:
- either by accident, when instructions end up with
  duplicate implicit operands (see e.g. D100939)
- or by design, when SIInstrInfo::copyPhysReg breaks a
  multi-dword copy into individual subreg mov instructions
  and adds implicit operands for the super-register.

The effect of this is that SIInstrInfo::isFoldableCopy can
be simplified and identifies more foldable copies. The test
diffs show that more immediate 0 values have been folded as
inline operands.

SIInstrInfo::isReallyTriviallyReMaterializable could
probably be simplified too but that is not part of this
patch.

Differential Revision: https://reviews.llvm.org/D114230