[AMDGPU][llvm-mc] Square-braced-syntax for registers - make ":expr2" optional.

Register numbers may be specified as assembly-time expressions.
This feature can be useful in macros and alike. However, expressions
are supported within sqare braces only.

Sqare braces were initially intended to support specifying of multiple
(pairs/quads...) registers. Syntax like v[8:8] which specifies single register
is also supported. That allows expressions but looks a bit unnatural.

This change supports syntax REG[EXPR].
Tests added.

Differential Revision: http://reviews.llvm.org/D20588

llvm-svn: 270990