Improvements for ReadyToRun helpers in JIT value numbering.

Improvements for ReadyToRun helpers in JIT value numbering.

The following ReadyToRun helpers are now treated similarly to their normal
counterparts in value numbering:
CORINFO_HELP_READYTORUN_NEW
CORINFO_HELP_READYTORUN_NEWARR_1
CORINFO_HELP_READYTORUN_ISINSTANCEOF
CORINFO_HELP_READYTORUN_CHKCAST
CORINFO_HELP_READYTORUN_STATIC_BASE

In particular, this allows CSE-ing calls to the last 3 of the above helpers when
possible. #3281 is an issue for CORINFO_HELP_READYTORUN_STATIC_BASE.

Compiler::fgValueNumberHelperCallFunc is refactored to reduce code
duplication.

Closes #3281.