MFV r304057:

MFV r304057:
7085 add support for "if" and "else" statements in dtrace

illumos/illumos-gate@c3bd3abd8856e8e75d820f65c58031cd6cbac818

Add syntactic sugar to dtrace: "if" and "else" statements. The sugar is
baked down to standard dtrace features by adding additional clauses with
the appropriate predicates.

Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Bryan Cantrill <bryan@joyent.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Matthew Ahrens <mahrens@delphix.com>

MFC after:	2 weeks
Relnotes:	yes