Add support for I32x4SConvertI16x8Low, I32x4SConvertI16x8High,
I32x4UConvertI16x8Low, I32x4UConvertI16x8High, I16x8SConvertI8x16Low,
I16x8SConvertI8x16High,I16x8SConvertI32x4, I16x8UConvertI32x4,
I16x8UConvertI8x16Low, I16x8UConvertI8x16High, I8x16SConvertI16x8,
I8x16UConvertI16x8 operations for mips32 and mips64 architectures.

Bug: 
Change-Id: I32f24956fc8e3c7df7f525bf0d4518161493a3ed
Reviewed-on: https://chromium-review.googlesource.com/517500
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Miran Karić <Miran.Karic@imgtec.com>
Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Cr-Commit-Position: refs/heads/master@{#46260}