[clr-interp] fix native int input to switch statements

[davidwrighton]

Fixes Runtime_68568 test

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[janvorli]

LGTM, thank you!

[Copilot]

Pull request overview

This PR fixes a bug in the CLR interpreter where native int (nint) values were not being properly converted when used as input to switch statements on 64-bit platforms. The fix adds a saturating conversion from 64-bit unsigned integers (U8) to 32-bit unsigned integers (U4), which ensures that native int values are properly clamped to the valid range expected by the switch instruction.

Key Changes

• Added new INTOP_CONV_U4_U8_SAT opcode for saturating U8→U4 conversion
• Compiler now emits this conversion on 64-bit targets when a StackTypeI (native int) is used with a switch statement
• Runtime implementation clamps values exceeding UINT32_MAX to UINT32_MAX before performing the conversion

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/interpreter/inc/intops.def	Defines the new INTOP_CONV_U4_U8_SAT opcode with standard 3-byte instruction format
src/coreclr/interpreter/compiler.cpp	Adds compile-time logic to emit saturating conversion for native int inputs to switch statements on 64-bit platforms
src/coreclr/vm/interpexec.cpp	Implements runtime behavior for the saturating conversion, clamping values to UINT32_MAX