RFR: 8266388: C2: Improve constant ShiftCntV on x86

[Vladimir Ivanov]

Clone ShiftCntV w/ a constant input during matching on x86. 
It enables `vshiftI_imm`/`vshiftL_imm` when ShiftCntV is shared.

Testing:
- [x] hs-tier1 - hs-tier4

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Commit messages:
 - 8266388: C2: Improve ShiftCntV with a constant shift on x86

Changes: https://git.openjdk.java.net/jdk/pull/3823/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3823&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266388
  Stats: 6 lines in 3 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3823.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3823/head:pull/3823

PR: https://git.openjdk.java.net/jdk/pull/3823