Sign-extend rather than zero-extend when promoting

Sign-extend rather than zero-extend when promoting
the condition for a BRCOND, according to what is
returned by getSetCCResultContents.  Since all
targets return the same thing (ZeroOrOneSetCCResult),
this should be harmless!  The point is that all over
the place the result of SETCC is fed directly into
BRCOND.  On machines for which getSetCCResultContents
returns ZeroOrNegativeOneSetCCResult, this is a
sign-extended boolean.  So it seems dangerous to
also feed BRCOND zero-extended booleans in some
circumstances - for example, when promoting the
condition.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@58861 91177308-0d34-0410-b5e6-96231b3b80d8