Replace Unsafe.As with Unsafe.BitCast in SpanHelpers.Fill

[jkotas]

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Pull Request Overview

This PR replaces all uses of Unsafe.As with Unsafe.BitCast in SpanHelpers.Fill<T> to streamline vector creation and improve performance.

• Removed the temporary tmp variable and switched to Unsafe.BitCast<T, U>(value) for all supported element sizes.
• Updated special-case branches for float and double to use BitCast.
• Simplified vector creation for 128-bit, 256-bit, and 512-bit cases by bit-casting value directly.

Comments suppressed due to low confidence (1)

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs:36

• Consider adding unit tests for Fill<T> covering each branch of Unsafe.BitCast for different T sizes to ensure the fill behavior remains correct across all cases.

vector = new Vector<byte>(Unsafe.BitCast<T, byte>(value));

[jkotas]

Context #111091 (comment)

[jkotas]

/ba-g timeouts, deadletter