Fix implementation of SDL instruction in MIPS64 simulator.
Use rt register as unsigned for shift operation.

TEST: cctest/test-run-wasm-64/RunWasmTurbofan_Store_i64_narrowed
Change-Id: I3a900631f8db401cf041e103436d4ec263a6f1c4
Reviewed-on: https://chromium-review.googlesource.com/856577Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#50439}