[DAGCombine] Remove SIGN_EXTEND-related inf-loop

The patch's author points out that, despite the function's documentation,
getSetCCResultType is only used to get the SETCC result type (with one
here-removed problematic exception). In one case, getSetCCResultType was being
used to get the predicate type to use for a SELECT node, and then
SIGN_EXTENDing (or truncating) to get the input predicate to match that type.
Unfortunately, this was happening inside visitSIGN_EXTEND, and creating new
SIGN_EXTEND nodes was causing an infinite loop. In addition, this behavior was
wrong if a target was not using ZeroOrNegativeOneBooleanContent. Lastly, the
extension/truncation seems unnecessary here: SELECT is defined as:

  Select(COND, TRUEVAL, FALSEVAL). If the type of the boolean COND is not i1
  then the high bits must conform to getBooleanContents.

So here we remove this use of getSetCCResultType and update
getSetCCResultType's documentation to reflect its actual uses.

Patch by deadal nix!