[15] RFR (M): 8238690: C2: Handle vector shifts by constant and non-constant scalar uniformly

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8238690/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8238690

Unify IR shape for vector shifts by scalar and always use:
    ShiftV src (ShiftCntV shift)

Right now, constant case has a special representation:
    ShiftV src ConI

It allows to simplify generic vector logic a bit and get rid of 
auxilliary chain rule on x86 (vshiftcntimm).

Testing: tier1-4

Best regards,
Vladimir Ivanov