(simplify_rtx): Don't do anything with if_then_else_cond result if both one arm…

(simplify_rtx): Don't do anything with if_then_else_cond result if both one arm and the input are a comparison.

(simplify_rtx): Don't do anything with if_then_else_cond
result if both one arm and the input are a comparison.
(simplify_{rtx,if_then_else,logical,shift_const}): Don't
test STORE_FLAG_VALUE with #if; properly test for just sign bit.
(num_sign_bit_copies, if_then_else_cond): Likewise.

From-SVN: r13551