[GISel] Erase the root instruction after emitting all its potential uses (#77494)

This tries to fix a bug by resolving a few FIXMEs. The bug is that
`EraseInstAction` is emitted after emitting the _first_ `BuildMIAction`,
which is too early because the erased instruction may still be used by
subsequent `BuildMIAction`s (in particular, by `CopyRenderer`).

An example of the bug (from `match-table-operand-types.td`):
```
def InstTest0 : GICombineRule<
  (defs root:$a),
  (match  (G_MUL i32:$x, i32:$b, i32:$c),
          (G_MUL $a, i32:$b, i32:$x)),
  (apply  (G_ADD i64:$tmp, $b, i32:$c),
          (G_ADD i8:$a, $b, i64:$tmp))>;

GIR_EraseFromParent, /*InsnID*/0,
GIR_BuildMI, /*InsnID*/1, /*Opcode*/GIMT_Encode2(TargetOpcode::G_ADD),
GIR_Copy, /*NewInsnID*/1, /*OldInsnID*/0, /*OpIdx*/0, // a
GIR_Copy, /*NewInsnID*/1, /*OldInsnID*/0, /*OpIdx*/1, // b
GIR_AddSimpleTempRegister, /*InsnID*/1, /*TempRegID*/0,
```

Here, the root instruction is destroyed before copying its operands ('a'
and 'b') to the new instruction.

The solution is to emit `EraseInstAction` for the root instruction as
the last action in the emission pipeline.