The round intrinsic gets translated to roundps on x86, which is SSE4.1
only. cvtps2pd + cvtdq2ps can be used as an SSE2 fallback. cvtps2pd
also corresponds to LLVM's nearbyint intrinsic.

BUG=swiftshader:20

Change-Id: I8b5896c443f202a5b25125b4e5049b0b3d3a11b0
Reviewed-on: https://chromium-review.googlesource.com/428491Reviewed-by: Jim Stichnoth <stichnot@chromium.org>