RFR: 8371162: Compiler warns about implicit cast from long to int in shift operation

[Archie Cobbs]

Consider code like this:

int x = 1;
x <<= 1L;

The compiler currently emits this warning:

warning: [lossy-conversions] implicit cast from long to int in compound assignment is possibly lossy
        x <<= 1L;
              ^

By definition, bit shift operations only use the bottom 5 or 6 bits of the specified shift amount (in this example, `1L`), and the JLS does not require the shift amount to be any specific integral type, only that that it be some integral type. So as long all but the bottom 5 or 6 bits are zero, there is no loss of information and the warning is inappropriate.

The case where the bottom 5 or 6 bits are _not_ all zero is addressed separately in [JDK-5038439](https://bugs.openjdk.org/browse/JDK-5038439).

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

Commit messages:
 - Avoid lossy conversion warnings for 64 bit shift amounts.

Changes: https://git.openjdk.org/jdk/pull/28180/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28180&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371162
  Stats: 44 lines in 2 files changed: 43 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28180.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28180/head:pull/28180

PR: https://git.openjdk.org/jdk/pull/28180