[Subzero][MIPS32] Implement sext, zext and trunc This patch adds support for sext, zext, trunc operations on i8, i16, i32 source operand types. Support for i1 source operand type will follow. R=stichnot@chromium.org Review URL: https://codereview.chromium.org/1948093002 . Patch from Sagar Thakur <sagar.thakur@imgtec.com>.