[HardwareLoops] Loop guard intrinsic to recognise zext

If a loop count was initially represented by a 32b unsigned int in C
then the hardware-loop pass can recognise the loop guard and insert
the llvm.test.set.loop.iterations intrinsic. If this was instead a
unsigned short/char then clang inserts a zext instruction to expand
the loop count to an i32. This patch adds the necessary pattern
matching to enable the use of lvm.test.set.loop.iterations in those
cases.

Patch by: sherwin-dc

Differential Revision: https://reviews.llvm.org/D109631