Error handling for malformed SPIRV input

[maarquitos14]

Further hardening of the translator against malformed input to prevent crashes. Specifically, this PR:

• Reports an error and invalidates module in SPIRVFunction::decode() for opcodes that don't belong in function scope.
• Adds null check with module invalidation and early exit after Decoder.getEntry() in SPIRVFunction::decodeBB() because it can return nullptr and it was otherwise dereferenced down the line.
• Reports an error and invalidates module for unknown extended instruction set IDs.
• Reports an error and invalidates module for truncated instructions.

5 new lit tests are added that were crashing before this patch. These new tests are a bit special, they need to feed deliberately malformed SPIR-V to the translator, including opcodes that don't exist, instructions in the wrong scope or truncated streams. Since spirv-as normally validates its input, the tests use its !<integer> syntax to inject raw 32-bit words directly into the binary output, bypassing validation. For example, !75535 emits the raw word (1<<16)|9999, which encodes a single-word instruction with unknown opcode 9999. Care must be taken with placement: ! words after variadic instructions (like OpTypeFunction) get absorbed as extra operands rather than emitted as standalone instructions.