Removed unnecessary MemoryMarshal.Cast call

[Sergio0694]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Optimization to the Block8x8 constructor, as discussed with @JimBobSquarePants.

Before (Release x64):

SixLabors.ImageSharp.Formats.Jpeg.Components.Block8x8..ctor(System.Span`1<Int16>)
    L0000: sub rsp, 0x28
    L0004: mov r8, [rdx]
    L0007: mov edx, [rdx+0x8]
    L000a: shl rdx, 1
    L000d: cmp rdx, 0x7fffffff
    L0014: ja L002a
    L0016: mov rdx, r8
    L0019: mov r8d, 0x80
    L001f: call 0x7ffa28883850
    L0024: nop
    L0025: add rsp, 0x28
    L0029: ret
    L002a: call 0x7ffa289aed50
    L002f: int3

After:

SixLabors.ImageSharp.Formats.Jpeg.Components.Block8x8..ctor(System.Span`1<Int16>)
    L0000: mov rdx, [rdx]
    L0003: mov r8d, 0x80
    L0009: call 0x7ffa28883850
    L000e: nop
    L000f: ret

[antonfirsov]

Relaunched the CI, we can merge as soon as all checks are passing.

[codecov]

Codecov Report

Merging #1117 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1117   +/-   ##
=======================================
  Coverage   82.10%   82.10%           
=======================================
  Files         680      680           
  Lines       28677    28677           
  Branches     3245     3245           
=======================================
  Hits        23544    23544           
  Misses       4457     4457           
  Partials      676      676           

Flag	Coverage Δ
#unittests	82.10% <100.00%> (ø)	⬆️

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