[llvm][Mips] Use a Target ISD opcode for PseudoD_SELECT (#84294)

The Mips target uses two TargetOpcode enumerators called
`PseudoD_SELECT_I` and `PseudoD_SELECT_I64`. A SDAG node is created
using these enumerators which is manually selected in
`MipsSEISelDAGToDAG.cpp` and ultimately expanded in
`EmitInstrWithCustomInserter` in `MipsISelLowering.cpp`.

This is not causing any upstream build to fail at the moment but it is
not guaranteed that these enumerators do not clash with Target ISD nodes
(i.e. those in the `MipsISD` namespace). We have seen this happening in
our downstream builds in which `Mips::PseudoD_SELECT_I` ends having the
same integer value as `MipsISD::VEXTRACT_ZEXT_ELT`. This confuses the
function `trySelect` in `MipsSEISelDAGToDAG.cpp` and causes a crash in 3
tests.

This change adds a new Target ISD opcode for these two cases and uses
them for the SDAG nodes. No test is included because this is a potential
error in the future not one that can be demonstrated in the current
codebase.