RFR: 8253594: Remove CollectedHeap::supports_tlab_allocation

[Stefan Karlsson]

CollectedHeap::supports_tlab_allocation always returns true, and is only used in an assert. I propose that we remove it.

GenCollectedHeap is the only implementation that didn't trivially return true. Instead it deferred the question to the
young gen, which after the CMS removal is only DefNew. I updated the GenCollectedHeap tlab functions to assert that the
young gen always supports TLABs. This could be taken even further, and the Generation::supports_tlab_allocation()
function could be entierly removed, but that requires that we change GenCollectedHeap to be explicit about the types
for _young_gen and _old_gen. So, I left  that part for another day.

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Commit messages:
 - 8253594: Remove CollectedHeap::supports_tlab_allocation

Changes: https://git.openjdk.java.net/jdk/pull/335/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=335&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253594
  Stats: 43 lines in 11 files changed: 0 ins; 37 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/335.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/335/head:pull/335

PR: https://git.openjdk.java.net/jdk/pull/335