Ivar reading only reads memory and doesn't modify
Print class name we read ivar from in JIT debugging

Make it easier to see what the class is we guard the ivar read against.
Setup self class for block based on creator info

This allows the JIT to inline ivar reads inside inlined blocks, for
example in cases like this:

def foo
  ary.each {|e| e == @bar }
end
Allow for compiled blocks to also have a receiver class

Before the additional data was either a block environment or a receiver
class, but having a receiver class is also useful for compiling blocks.
This matters for example for inlining instance variable reads if just a
block is compiled.

Improves benchmark/tiers/0/bm_vm1_ivar.rb significantly:

Before:
=== bin/rbx ===
  ivar read 58492061.6 (±3.0%) i/s -  289205872 in   4.948913s
   ivar set 41165744.5 (±2.6%) i/s -  204915000 in   4.982051s

After:
=== bin/rbx ===
  ivar read 266371814.2 (±5.6%) i/s - 1283690000 in   4.836606s
   ivar set 224551559.4 (±4.0%) i/s - 1110346272 in   4.953298s