[BUG] Compiler optimizations can prevent bounds checks

[WardBrian]

Current Behavior:

Enabling compiler optimizations currently stops emitting a default assignment for a variable if it is immediately assigned to (#1029). However, this changes the behavior of those very assign functions when it comes to bounds checks:

https://github.com/stan-dev/stan/blob/4ec109a21e5bd1cd43eed724ef286aadec3b5e7a/src/stan/model/indexing/access_helpers.hpp#L58

This means the same model:

parameters {
  vector[3] y;
}

model {
  vector[2] x;
  x = y;
  y ~ std_normal();
  print(x); // prevent dead-code-elimination on x
}

Will throw an error at --O0:

Unrecoverable error evaluating the log probability at the initial value.
Exception: vector assign rows: assigning variable x (2) and right hand side rows (3) must match in size (in '../../ml/stanc3/bounds.stan', line 12, column 4 to column 10)
Exception: vector assign rows: assigning variable x (2) and right hand side rows (3) must match in size (in '../../ml/stanc3/bounds.stan', line 12, column 4 to column 10)

But will sample to completion at --O1.

Expected Behavior:

Bounds checks are applied uniformly.