RFR: 8256655: rework long counted loop handling

[Roland Westrelin]

Currently the transformation of a long counted loop into a loop nest
with an inner int counted loop is performed in 2 steps:

1- recognize the counted loop shape and build the loop nest
2- transform the inner loop into a counted loop

I propose changing this to a 3 steps process:

1- recognize the counted loop shape (transform the loop into a LongCountedLoop)
2- build the loop nest
3- transform the inner loop into a counted loop

The benefits are:

- the logic is cleaner because step 1 and 2 are now separated
- Simple optimizations (loop iv type, empty loop elimination, parallel
  iv) can be implemented for LongCountedLoop by refactoring existing
  code

1- above is achieved by refactoring the
PhaseIdealLoop::is_counted_loop() so it takes an extra parameter (the
kind of counted loop, int or long).

2- is the existing loop nest construction logic. But now that it takes
a LongCountedLoop as input, the shape of the loop is known to be that
of a canonicalized counted loop. As a result, the loop nest
construction is simpler.

This change also refactors PhiNode::Value() so that it works for both
CountedLoop and LongCountedLoop.

I also changed ConvL2INode to be a TypeNode (ConvI2LNode is a type
node) and:

    jlong init_p = (jlong)init_t->_lo + stride_con;
    if (init_p > (jlong)max_jint || init_p > (jlong)limit_t->_hi)
      return false; // cyclic loop or this loop trips only once

to:

    if (init_t->lo_as_long() > max_signed_integer(iv_bt) - stride_con) {

because if the loop has a single iteration transforming it to a
CountedLoop should allow the backedge to be optimized out.

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

Commit messages:
 - fix trailing whitespace
 - fix comment
 - long counted loop refactoring

Changes: https://git.openjdk.java.net/jdk/pull/1316/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1316&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256655
  Stats: 967 lines in 25 files changed: 508 ins; 211 del; 248 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1316.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1316/head:pull/1316

PR: https://git.openjdk.java.net/jdk/pull/1316