[hwasan] Omit tag check for null pointers (#122206)
If the pointer to be checked is statically known to be zero, the tag
check will always pass since:
1) the tag is zero
2) shadow memory for address 0 is initialized to 0 and never updated.
We can therefore elide the tag check.
We perform the elision in two places:
1) the HWASan pass
2) when lowering the CHECK_MEMACCESS intrinsic. Conceivably, the HWASan
pass may encounter a "cannot currently statically prove to be null"
pointer (and is therefore unable to omit the intrinsic) that later
optimization passes convert into a statically known-null pointer. As a
last line of defense, we perform elision here too.
This also updates the tests from
https://github.com/llvm/llvm-project/pull/122186
