Fix UBSAN in __ubsan::Value::getSIntValue

/home/marxin/Programming/gcc2/libsanitizer/ubsan/ubsan_value.cpp:77:25: runtime error: left shift of 0x0000000000000000fffffffffffffffb by 96 places cannot be represented in type '__int128'
    #0 0x7ffff754edfe in __ubsan::Value::getSIntValue() const /home/marxin/Programming/gcc2/libsanitizer/ubsan/ubsan_value.cpp:77
    #1 0x7ffff7548719 in __ubsan::Value::isNegative() const /home/marxin/Programming/gcc2/libsanitizer/ubsan/ubsan_value.h:190
    rust-lang#2 0x7ffff7542a34 in handleShiftOutOfBoundsImpl /home/marxin/Programming/gcc2/libsanitizer/ubsan/ubsan_handlers.cpp:338
    rust-lang#3 0x7ffff75431b7 in __ubsan_handle_shift_out_of_bounds /home/marxin/Programming/gcc2/libsanitizer/ubsan/ubsan_handlers.cpp:370
    rust-lang#4 0x40067f in main (/home/marxin/Programming/testcases/a.out+0x40067f)
    rust-lang#5 0x7ffff72c8b24 in __libc_start_main (/lib64/libc.so.6+0x27b24)
    rust-lang#6 0x4005bd in _start (/home/marxin/Programming/testcases/a.out+0x4005bd)

Differential Revision: https://reviews.llvm.org/D97263

compiler-rt/lib/ubsan/ubsan_value.cpp

@@ -74,7 +74,7 @@ SIntMax Value::getSIntValue() const {
     // to SIntMax.
     const unsigned ExtraBits =
       sizeof(SIntMax) * 8 - getType().getIntegerBitWidth();
-    return SIntMax(Val) << ExtraBits >> ExtraBits;
+    return SIntMax(UIntMax(Val) << ExtraBits) >> ExtraBits;
   }
   if (getType().getIntegerBitWidth() == 64)
     return *reinterpret_cast<s64*>(Val);