[RISC-V] Restore temp register in switch table codegen

[tomeksowi]

Regression after #117048, using idxReg as temp clobbered the value if it was used later

Part of #84834, cc @dotnet/samsung

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[tomeksowi]

@risc-vv /run

[risc-vv]

RISC-V pull_request-CLR-QEMU: 9097 / 9127 (99.67%)

=======================
      passed: 9097
      failed: 2
     skipped: 593
      killed: 28
------------------------
 TOTAL tests: 9720
VIRTUAL time: 80h 28min 33s 746ms
   REAL time: 1h 18min 51s 777ms
=======================

report.xml, report.md, failures.xml, testclr_details.tar.zst

RISC-V pull_request-CLR-VF2: 9096 / 9127 (99.66%)

=======================
      passed: 9096
      failed: 3
     skipped: 593
      killed: 28
------------------------
 TOTAL tests: 9720
VIRTUAL time: 32h 7min 4s 92ms
   REAL time: 2h 7min 32s 706ms
=======================

report.xml, report.md, failures.xml, testclr_details.tar.zst

RISC-V pull_request-FX-QEMU: 0 / 1 (0.00%)

=======================
      passed: 0
      failed: 0
     skipped: 14
      killed: 1
------------------------
 TOTAL tests: 15
VIRTUAL time: 16s 222ms
   REAL time: 4h 30min 0s 478ms
=======================

report.xml, report.md, failures.xml, testclr_details.tar.zst

RISC-V pull_request-FX-VF2: 0 / 54 (0.00%)

=======================
      passed: 0
      failed: 0
     skipped: 14
      killed: 54
------------------------
 TOTAL tests: 68
VIRTUAL time: 1min 11s 4ms
   REAL time: 13h 0min 14s 28ms
=======================

report.xml, report.md, failures.xml, testclr_details.tar.zst

Build information and commands

GIT: a70d1898476b167b40f86959a3f8d4a96117c772
CI: d023c7d591ab286aa343d644f00b5daa59d8d6b0
REPO: dotnet/runtime
BRANCH: main
CONFIG: Checked
LIB_CONFIG: Release