[RyuJIT] Unify handling of nodes that produce multiple registers

[CarolEidt]

Nodes that produce multiple registers require special handling. There should be a unified mechanism for manipulating the register and spill info on these nodes. The nodes are:

• GenTreeMultiRegOp (_TARGET_ARM_)
• GenTreePutArgSplit (_TARGET_ARM_)
• GenTreeCall (FEATURE_MULTIREG_RET)

See the declarations of the following methods on these nodes:

• ClearOtherRegs()
• ClearOtherRegFlags()
• GetRegNumByIdx()
• GetRegSpillFlagByIdx()
• SetRegSpillFlagByIdx()
• GetRegType()

category:implementation
theme:ir
skill-level:expert
cost:medium

[CarolEidt]

When this is addressed, we should also fix the assert in LinearScan::BuildRefPositionsForNode() that checks that produce is as expected - that assert should be generalized for all targets.