[msan] Switch switch() from strict handling to (icmp eq)-style handling (#179851)

Currently, the SwitchInst:
```
switch i32 %Val, label %else [ i32 0, label %A
                               i32 1, label %B
                               i32 2, label %C ]
```
is strictly handled i.e., MSan will check that %Val is fully
initialized. This is appropriate nearly all the time.

However, sometimes the compiler may convert (icmp + br) into a switch
statement. (icmp + br) has different semantics: MSan allows icmp eq/ne
with partly initialized inputs to still result in a fully initialized
output, if there exists a bit that is initialized in both inputs with a
differing value e.g., suppose:
```
%A   = 00000000 00001010
%B   = 00000000 00000110
%C   = 00000000 00000011

%Val = 00000001 ???????? (where ? denotes an uninitialized bit)
```
Even though %Val has uninitialized bits, the initialized '1' bit
immediately to the left, compared to the corresponding initialized '0'
bit in %A/%B/%C suffices to prove that %Val does not match any of those
cases. This is similar to a real-world case with std::optional (where
the has_value bit may be initialized but the value is not).

This patch adds this relaxed icmp logic to the switch instrumentation as
well, to make MSan's behavior equivalent under optimization.

Note that this edge case only applies if the switch input value
definitively does not match *any* of the cases (matching any of the
cases requires an exact, fully initialized match). If it is uncertain
whether the switch input value could, depending on the uninitialized
bits, match one of the cases or not, MSan will report
use-of-uninitialized memory.