[tblgen] Fix undefined behaviour when assigning integers to large bits<n>'s

This code:
  bits<96> X = 0;
was triggering undefined behaviour since it iterates over bits 0..95 and tests
them against the IntInit using 1LL << I.

This patch resolves the undefined behaviour by continuing to treat the IntInit
as a 64-bit value and simply causing all bit tests in excess of 64-bits to report
false. As a result,
  bits<96> X = -1;
will be equivalent to:
  bits<96> X;
  let X{0-63} = -1;
  let X{64-95} = 0;

llvm-svn: 342744