ggml : fix UB in IQ2_S and IQ3_S

[ggerganov]

Running on master with thread-sanitizer enabled produces UB warnings:

./bin/test-quantize-fns

Testing iq3_s
/Users/ggerganov/development/github/llama.cpp/ggml-quants.c:9389:79: runtime error: left shift of 36408 by 16 places cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Users/ggerganov/development/github/llama.cpp/ggml-quants.c:9389:79 in 
/Users/ggerganov/development/github/llama.cpp/ggml-quants.c:9398:79: runtime error: left shift of 34588 by 16 places cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Users/ggerganov/development/github/llama.cpp/ggml-quants.c:9398:79 in 
Testing iq2_s
/Users/ggerganov/development/github/llama.cpp/ggml-quants.c:9028:79: runtime error: left shift of 36408 by 16 places cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Users/ggerganov/development/github/llama.cpp/ggml-quants.c:9028:79 in 
/Users/ggerganov/development/github/llama.cpp/ggml-quants.c:9037:79: runtime error: left shift of 34588 by 16 places cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Users/ggerganov/development/github/llama.cpp/ggml-quants.c:9037:79 in 

I doubt this causes any real issues, but fixing it to make the thread sanitizer happy

[ikawrakow]

Did the UB sanitizer check out the assembly just in case?

It is funny because some compilers cannot be forced to do the shift on 16-bit ints, no matter what you do. Notably, the Metal compiler is one of those.