Free the second half of double register (#46939)

* free the second half of double register

* Take appropriate regmask

* revert part of comment

Author: kunalspathak

src/coreclr/jit/lsra.cpp

@@ -4934,6 +4934,20 @@ void LinearScan::processBlockStartLocations(BasicBlock* currentBlock)
                     unassignIntervalBlockStart(getSecondHalfRegRec(targetRegRecord),
                                                allocationPassComplete ? nullptr : inVarToRegMap);
                 }
+
+                // If this is a TYP_FLOAT interval, and the assigned interval was TYP_DOUBLE, we also
+                // need to update the liveRegs to specify that the other half is not live anymore.
+                // As mentioned above, for TYP_DOUBLE, the other half will be unassigned further below.
+                if ((interval->registerType == TYP_FLOAT) &&
+                    ((targetRegRecord->assignedInterval != nullptr) &&
+                     (targetRegRecord->assignedInterval->registerType == TYP_DOUBLE)))
+                {
+                    RegRecord* anotherHalfRegRec = findAnotherHalfRegRec(targetRegRecord);
+
+                    // Use TYP_FLOAT to get the regmask of just the half reg.
+                    liveRegs &= ~getRegMask(anotherHalfRegRec->regNum, TYP_FLOAT);
+                }
+
 #endif // TARGET_ARM
                 unassignIntervalBlockStart(targetRegRecord, allocationPassComplete ? nullptr : inVarToRegMap);
                 assignPhysReg(targetRegRecord, interval);