[Mips] Remove a trivial variable (NFC)

[kazutakahirata]

We assign I->getNumOperands() to J and immediately print that out as a
debug message. We don't need to keep J across iterations.

[llvm-ci]

LLVM Buildbot has detected a new failure on builder clang-arm64-windows-msvc running on linaro-armv8-windows-msvc-04 while building llvm at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/161/builds/1536

Here is the relevant piece of the build log for the reference:

Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'Clang :: Modules/hashing-decls-in-exprs-from-gmf.cppm' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
rm -rf C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\stage1\tools\clang\test\Modules\Output\hashing-decls-in-exprs-from-gmf.cppm.tmp
# executed command: rm -rf 'C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\stage1\tools\clang\test\Modules\Output\hashing-decls-in-exprs-from-gmf.cppm.tmp'
# RUN: at line 2
mkdir -p C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\stage1\tools\clang\test\Modules\Output\hashing-decls-in-exprs-from-gmf.cppm.tmp
# executed command: mkdir -p 'C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\stage1\tools\clang\test\Modules\Output\hashing-decls-in-exprs-from-gmf.cppm.tmp'
# RUN: at line 3
split-file C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\llvm\clang\test\Modules\hashing-decls-in-exprs-from-gmf.cppm C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\stage1\tools\clang\test\Modules\Output\hashing-decls-in-exprs-from-gmf.cppm.tmp
# executed command: split-file 'C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\llvm\clang\test\Modules\hashing-decls-in-exprs-from-gmf.cppm' 'C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\stage1\tools\clang\test\Modules\Output\hashing-decls-in-exprs-from-gmf.cppm.tmp'
# .---command stderr------------
# | split-file: error: C:\Users\tcwg\llvm-worker\clang-arm64-windows-msvc\llvm\clang\test\Modules\hashing-decls-in-exprs-from-gmf.cppm: permission denied
# `-----------------------------
# error: command failed with exit status: 1

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