RFR: JDK-8303154: Investigate and improve instruction cache flushing during compilation

[Damon Fenacci]

It was noticed that we flush the instruction cache too much for a single C1 compilation. The same is true for the C2 compilation.
There are several places in the code where the instruction cache is called and many of them are very intertwined (see [bug report](https://bugs.openjdk.org/browse/JDK-8303154)).

This PR is meant to be a "minimum" set of changes that improve the situation without introducing excessive extra information to keep track of the origin of the call through call stacks. This is done by avoiding calls to flush the ICache at `Compilation::emit_code_epilog` and  when copying from the temporary buffer into the code cache in `CodeBuffer::copy_code_to` if there is a call to `CodeCache::commit` later on. This results in flushing the ICache only once instead of 3 times for a C1 compilation and twice for a C2 compilation respectively.

This change decreases the number of calls to flush the ICache for a simple _Hello world_ program on Mac OSX aarch64 from 3568 to 3487 on C1 (2.27% improvement) and from  3499 to 3415 on C2 (2.40% improvement).

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Commit messages:
 - JDK-8303154: fix syntax and add comment
 - JDK-8303154: fix syntax
 - JDK-8303154: remove unnecessary icache flushing at the end emit_code_epilog
 - JDK-8303154: Investigate and improve instruction cache flushing during compilation

Changes: https://git.openjdk.org/jdk/pull/12877/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12877&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303154
  Stats: 15 lines in 5 files changed: 4 ins; 3 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/12877.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12877/head:pull/12877

PR: https://git.openjdk.org/jdk/pull/12877