Pretty print i1 values using boolean names

[TomMD]

In at least llvm 3.8 the values of type i1 appear as true or false instead of 18446744073709551615 (which seems odd for a "1 bit" value). We should make the printing of these literals take into account the type.

For example

$ cat t.c
int f(int x)
{
    return (x == 0 || x == 42);
}
$ clang -emit-llvm -c t.c -o t.bc
$ llvm-dis t.bc ; cat t.ll | grep phi
  %8 = phi i1 [ true, %0 ], [ %6, %4 ]
$ llvm-disasm t.bc | grep phi
  %8 = phi i1 [ 18446744073709551615, %0 ], [ %6, %4 ]