Subzero: Fix sext/zext lowering with i1 source operands.

Also declare a few variables as Constant* instead of Operand* when they hold the result of Ctx->getConstantInt(), to be consistent with the rest of the code.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/541093002
3 files changed