[TblGen] Extend !if semantics through new feature !cond
This patch extends TableGen language with !cond operator.
Instead of embedding !if inside !if which can get cumbersome,
one can now use !cond.
Below is an example to convert an integer 'x' into a string:
!cond(!lt(x,0) : "Negative", !eq(x,0) : "Zero", !eq(x,1) : "One, 1 : "MoreThanOne")
Reviewed By: hfinkel, simon_tatham, greened
Differential Revision: https://reviews.llvm.org/D55758