[RISCV] Fix crash in combinePExtTruncate for truncate(srl) without MUL/SUB (#186141)
combinePExtTruncate is called from performTRUNCATECombine when the
P-extension is enabled. It attempts to match patterns like
truncate(srl(mul/sub(...), shamt)) and combine them into P-extension
narrowing shift instructions (e.g. PNSRLI, PNSRAI).
However, after extracting the shift input operand `Op` from the SRL
node, the function unconditionally accessed Op.getOperand(0) and
Op.getOperand(1) without first verifying that Op has at least two
operands. For example, when combining:
```
truncate(v2i16
srl(v2i32
bitcast(v2i32 i64), <-- Op = bitcast, a unary op with 1 operand
BUILD_VECTOR <8, 8>))
```
Op is a BITCAST node (unary, only 1 operand), so accessing
Op.getOperand(1) triggers an out-of-bounds assertion:
```
Assertion `Num < NumOperands && "Invalid child # of SDNode!"' failed.
```
Add an early return when Op has fewer than two operands.
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