Suppress the "conversion from 'size_t' to 'type', possible loss of data"
warning which gets treated as an error, to match the CMake build for
MSVC, and other compilers.

Bug: swiftshader:131
Change-Id: I2fb6695e5ed541b052e170b577fcaafb6ba169c4
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34268
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>