Fixed 2 issues:
- Added missing VK_FORMAT_R16* formats in SamplerCore
- Made sure the minimum signed EAC value also gets bit shifted

Bug b/119620767

Change-Id: I3189001316cc0c65d3be9a9bd2e986df4f7a3690
Tests: dEQP-VK.*eac*
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30908Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>