RFR: 8262916: Merge LShiftCntV and RShiftCntV into a single node

[Eric Liu]

The vector shift count was defined by two separate nodes(LShiftCntV and
RShiftCntV), which would prevent them from being shared when the shift
counts are the same.

public static void test_shiftv(int sh) {
    for (int i = 0; i < N; i+=1) {
        a0[i] = a1[i] << sh;
        b0[i] = b1[i] >> sh;
    }
}

Given the example above, by merging the same shift counts into one
node, they could be shared by shift nodes(RShiftV or LShiftV) like
below:

Before:
1184  LShiftCntV  === _  1189  [[ 1185  ... ]]
1190  RShiftCntV  === _  1189  [[ 1191  ... ]]
1185  LShiftVI  === _  1181  1184  [[ 1186 ]]
1191  RShiftVI  === _  1187  1190  [[ 1192 ]]

After:
1190  ShiftCntV  === _  1189  [[ 1191 1204  ... ]]
1204  LShiftVI  === _  1211  1190  [[ 1203 ]]
1191  RShiftVI  === _  1187  1190  [[ 1192 ]]

The final code could remove one redundant “dup”(scalar->vector),
with one register saved.

Before:
        dup     v16.16b, w12
        dup     v17.16b, w12
        ...
        ldr     q18, [x13, #16]
        sshl    v18.4s, v18.4s, v16.4s
        add     x18, x16, x12           ; iaload

        add     x4, x15, x12
        str     q18, [x4, #16]          ; iastore

        ldr     q18, [x18, #16]
        add     x12, x14, x12
        neg     v19.16b, v17.16b
        sshl    v18.4s, v18.4s, v19.4s
        str     q18, [x12, #16]         ; iastore

After:
        dup	v16.16b, w11
        ...
        ldr	q17, [x13, #16]
        sshl	v17.4s, v17.4s, v16.4s
        add	x2, x22, x11            ; iaload

        add	x4, x16, x11
        str	q17, [x4, #16]          ; iastore

        ldr	q17, [x2, #16]
        add	x11, x21, x11
        neg	v18.16b, v16.16b
        sshl	v17.4s, v17.4s, v18.4s
        str	q17, [x11, #16]         ; iastore

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

Commit messages:
 - 8262916: Merge LShiftCntV and RShiftCntV into a single node

Changes: https://git.openjdk.java.net/jdk/pull/3371/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3371&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262916
  Stats: 1494 lines in 14 files changed: 988 ins; 282 del; 224 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3371.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3371/head:pull/3371

PR: https://git.openjdk.java.net/jdk/pull/3371