Perform gc-state checks with LoadB to fit C2 matchers

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/c2-match-gc-state/webrev.01/

Since we picked up the fix for matchers [1], all our precious LoadUB for gc-state loads now mismatch
again. We can replace them with LoadB to match the other two matchers from upstream. Patch converts
all LoadUBs for gc_state to LoadB, asserts the shape is correct, and blocks LoadB->LoadUB conversion
in AddINode::Ideal.

Sample WB check before:

   ...
       0x00007fa89ccf0254: movzbl 0x20(%r15),%r11d
       0x00007fa89ccf0259: test   %r11d,%r11d
  ╭    0x00007fa89ccf025c: jne    0x00007fa89ccf0278
  │    ...
  ↘    0x00007fa89ccf0278: movzbl 0x20(%r15),%r11d
       0x00007fa89ccf027d: test   $0x14,%r11d
       0x00007fa89ccf0284: jne    0x00007fa89ccf02d2
       ...

Sample WB check after:

       0x00007f3940de31b4: cmpb   $0x0,0x20(%r15)      ; heap-stable check
    ╭  0x00007f3940de31b9: jne    0x00007f3940de31dd
    │   ...
    ↘  0x00007f3940de31dd: testb  $0x14,0x20(%r15)     ; wb slowpath, evac-enabled check
       0x00007f3940de31e2: jne    0x00007f3940de33ce


Testing: tier3_gc_shenandoah, eyeballing assembly

[1] https://bugs.openjdk.java.net/browse/JDK-8204479