[mips][ias] fix li macro when values are negated with ~

The integrated assembler evaluates the expressions such as ~0x80000000 to
0xffffffff7fffffff early in the parsing process. This patch adds compatibility
with gas so that li loads the expected value (0x7fffffff) in those cases. This
only occurs iff all the upper 32bits are set and maintains existing checks by
not truncating the result down to 32 bits if any of the the upper bits are not
set.

Reviewers: dsanders, zoran.jovanovic

Differential Review: https://reviews.llvm.org/D23399

llvm-svn: 283353