This change fixes below errors:
error X3013: 'asuint': no matching 1 parameter intrinsic function
error X3013: Possible intrinsic functions are:
error X3013:     asuint(double, out uint x, out uint y)
error X3013:     asuint(float|half|int|uint)

When we store a boolean value, we should cast it to uint firstly. Then do the
store.

Bug: angleproject:1951

Change-Id: I8931e540d701ef72bb82846496c698acef1ba1fb
Reviewed-on: https://chromium-review.googlesource.com/c/1295831
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>