Refactor register arg handling in TreeNodeInfoInit.

Refactor register arg handling in TreeNodeInfoInit.

The handling for outgoing register arguments in TreeNodeInfoInitCall was
unnecessarily complex and difficult to follow. In particular, the processing
of FIELD_LIST arguments performed a fair amount of unnecessary work,
contained code to deal with impossible IR patterns, and did not share any
code for handling the component PUTARG_REG nodes with the normal PUTARG_REG
case. This changes resolves all three of these issues and fixes an ordering
issue in lowering that was causing PInvoke prologs to remain unlowered.