[RISC-V] Simplifying the loop generated in genZeroInitFrameUsingBlockInit

[sirntar]

This PR is a resubmission of #114003 after fixing some overflow problems. I decided to drop jump encoding changes for this PR to avoid mixing two separate problems (I will resubmit the second part in the future).

As I mentioned in #114003 , a partial unrolling of the loop (and removing rCnt) results in a small performance gain (1.4% on average for coreclr tests and about 0.5% for corefx tests).

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Part of #84834, cc @dotnet/samsung

[risc-vv]

988e2bb is being scheduled for building and testing

GIT: 988e2bb20b4498427c8a5b9c092b2cadf3b0c69c
REPO: dotnet/runtime
BRANCH: main

[risc-vv]

RISC-V Release-CLR-QEMU: 9075 / 9105 (99.67%)

=======================
      passed: 9075
      failed: 2
     skipped: 597
      killed: 28
------------------------
 TOTAL tests: 9702
VIRTUAL time: 35h 13min 10s 960ms
   REAL time: 36min 3s 53ms
=======================

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

Build information and commands

GIT: 52e7c3de32f3282348b7ee59c09ae3ec062b630e
CI: 8d15504ae1358a01693a7c50a10ed6536bf11989
REPO: dotnet/runtime
BRANCH: main
CONFIG: Release
LIB_CONFIG: Release