[mips][ias] Fix all the hacks related to MIPS-specific unary operators (%hi/%lo/...

[mips][ias] Fix all the hacks related to MIPS-specific unary operators (%hi/%lo/%gp_rel/etc.).

Summary:
They are now lexed as a single token on targets where
MCAsmInfo::HasMipsExpressions is true and then parsed in a similar way to
the '~' operator as part of MCExpr::parseExpression.

As a result:
* expressions and immediates no longer have different parsing rules. The
  difference is now solely down to whether evaluateAsAbsolute() succeeds.
* %hi(%neg(%gp_rel(x))) are no longer parsed as a single operator and
  decomposed into the three MipsMCExpr nodes. They are parsed directly as
  three MipsMCExpr nodes.
  * parseMemOperand no longer needs to eat all the surrounding parenthesis
    to get at the outermost operator to make this work
* %hi(%neg(%gp_rel(x))) and %lo(%neg(%gp_rel(x))) are no longer the only
  3-in-1 relocs that parse for N64. They're still the only combinations that
  are permitted in relocatable expressions though. Fixing that should be a
  later patch.
* We no longer need to list all the tokens that can occur as the first token of
  an expression or immediate.

test/MC/Mips/expr1.s:
    This change also prevents the incorrect lowering of %lo(2*4)+foo to
    %lo(8+foo) which is not an equivalent expression (the difference is
    whether foo is truncated to 16-bit or not) and the test has been
    updated to account for the macro expansion the correct expression requires.

Reviewers: sdardis

Subscribers: dsanders, sdardis, llvm-commits

Differential Revision: https://reviews.llvm.org/D23110

llvm-svn: 277988