compiler: remove LHS/RHS context determination for var exprs

Remove the code that determined LHS/RHS context for var expressions
(was needed previously for some instances of the back end). LHS/RHS
context is computed by the back end in all cases, so no need to have
this code in the front end any more.

Change-Id: Iee8b5d2bb35ef529caeb27e985da49646fb1676d
Reviewed-on: https://go-review.googlesource.com/77510
Reviewed-by: Ian Lance Taylor <iant@golang.org>
8 files changed