RFR: 8297539: Consolidate the uses of the int<->float union conversion trick [v2]

[Afshin Zafari]

> ### Discussion
> 
> There are different ways of casting/converting integral types to floating point types and vice versa in the code. 
> These conversions are changed to invocation of a centralised `cast<To>(From)` method in `primitiveConversions.hpp`. To make this changes build and run, some new `cast` methods are introduced and header dependencies changed. 
> 
> ### Patch
> 
> - All instances of using `union` technique that are used for int<->float and long<->double type conversions are replaced by `PrimitiveConversions::cast<To>(From)` method.
>   - Instances of, for example, converting `int`<->`intptr_t` are not changed.
> - After this change, `JavaValue` also uses the `PrimitiveConversions::cast<To>(From)` templates. Therefore, all the `union` conversions for `int`<-> `float` and `long` <-> `double` in hotspot are now centralised in one location.
> - Templates in `PrimitiveConversions` class are numbered from 1-9 and all lines of code that use `PrimitivEConversions::cast<To>(From)` have comments saying which templated `cast<To>(From)` matches. Issue number 8297539 is also in the comments to make the search/reviews more easier.
> - The new 'cast<To>(From)' templates are numbered 6-9 for some instances where existing templates (1-5) did not match.
> - `globalDefinitions.hpp` now includes `primitiveConversions.hpp`.
> - `primitiveConversions.hpp` does NOT include `globalDefinitions.hpp` any more. Some typedef's are added to it to compensate the missing types.
> - The followings are not changed:
>   - `jni.hpp`
>   - `ciConstant.hpp` is not changed, since uinon is not used for converting `int` <->`float` or `long` <-> `double`. There are some asserts that allow setter/getter used only for the same types.
>   - `bytecodeInterpreter_zero.hpp` is not changed. Writing to and reading from the `VMJavaVal64` are always with the same type and no conversion is made.
>   - union with bit fields are not changed.
>   - `json.hpp` and `json.cpp` in src/hotspot/share/utilities.
> 
> ### Test
> mach5 tiers 1-5 passed, tiers 6-8 in progress

Afshin Zafari has updated the pull request incrementally with one additional commit since the last revision:

  8297539: Consolidate the uses of the int<->float union conversion trick

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12384/files
  - new: https://git.openjdk.org/jdk/pull/12384/files/33d1c8ec..653f93b1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12384&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12384&range=00-01

  Stats: 388 lines in 18 files changed: 211 ins; 139 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/12384.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12384/head:pull/12384

PR: https://git.openjdk.org/jdk/pull/12384