Re-examine use of CodeBuffer::verify_section_allocation

JDK-7104960 added product-enabled verification of CodeBuffer sections via verify_section_allocation, run when constructing, expanding and tearing down CodeBuffers. This has some measurable overhead, especially for routines like PhaseOutput::scratch_emit_size. About 200k instructions are executed on a Hello World in this verification.

To reduce overhead, but retain most of the verification guarantees, we could consider making all calls debug-only, except the one in ~CodeBuffer.