Added check for temporary register overflow

Added check for temporary register overflow

Currently, when overflowing the number of temporary registers
allowed, SwiftShader will crash somewhere in the generated
code. This cl adds an early check to prevent the crash and
instead output an error message to the user.

Bug chromium:814987

Change-Id: Idadda21ee14298662763d986ee1283a5114c1c01
Reviewed-on: https://swiftshader-review.googlesource.com/18388
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>