integer: remove explicit casts from _MIN definitions

The spec says (section 6.12.3, CL version 1.2):
  The macro names given in the following list must use the values
  specified. The values shall all be constant expressions suitable
  for use in #if preprocessing directives.

This commit addresses the second part of that statement.

Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Tom Stellard <tom@stellard.net>
CC: Moritz Pflanzer <moritz.pflanzer14@imperial.ac.uk>
CC: Serge Martin <edb+libclc@sigluy.net>
llvm-svn: 249445