[lworld+fp16] RFR: 8336406: Add support for FP16 binary operations [v2]

[Bhavana Kilambi]

> This patch adds middle end support in C2 for a few FP16 binary operations, namely - subtract, multiply, divide, min and max. It also adds aarch64 backend support for these operations.
> 
> Tested JTREG tests -
> hotspot/jtreg/compiler/vectorization/TestFloat16VectorOps.java, hotspot/jtreg/compiler/vectorization/TestFloat16VectorReinterpretConv.java, hotspot/jtreg/compiler/intrinsics/float16 and test/jdk/java/lang/Float16 and they successfully pass on aarch64 and x86 machines.

Bhavana Kilambi has updated the pull request incrementally with one additional commit since the last revision:

  Add MulHF::Ideal() and MulHF Idealization tests

-------------

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1175/files
  - new: https://git.openjdk.org/valhalla/pull/1175/files/525c3530..914dfc24

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1175&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1175&range=00-01

  Stats: 88 lines in 3 files changed: 85 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/valhalla/pull/1175.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1175/head:pull/1175

PR: https://git.openjdk.org/valhalla/pull/1175