[SILoadStoreOptimizer] Use std::abs to avoid truncation.

Using regular abs() causes the following warning

error: absolute value function 'abs' given an argument of type 'int64_t' (aka 'long') but has parameter of type 'int' which may cause truncation of value [-Werror,-Wabsolute-value]
        (uint32_t)abs(Dist) > MaxDist) {
                  ^
lib/Target/AMDGPU/SILoadStoreOptimizer.cpp:1369:19: note: use function 'std::abs' instead

which causes a bot to fail:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux/builds/18284/steps/bootstrap%20clang/logs/stdio

llvm-svn: 349224