vpx_dsp/inv_txfm: fix int sanitizer warnings
vpx_dsp/inv_txfm: fix int sanitizer warnings

with vp9-highbitdepth off.
Unit Test:  SSE2/Trans16x16DCT , VP9/LevelTest.TestTargetLevel20Large, VP9/CpuSpeedTest

implicit conversion from type 'int32_t' (aka 'int') of value -32851
(32-bit, signed) to type 'tran_low_t' (aka 'short') changed the value to
32685 (16-bit, signed)

BUG=webm:1615
BUG=webm:1647

Change-Id: I9ef064dc9ac734379628565ff6505b0876984123

ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx