For whole-image 4x8-bit normalized format multisample resolves, use a
specialized code path instead of a generic blit routine.

Benchmark results:

Run on (48 X 2594 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x24)
  L1 Instruction 32 KiB (x24)
  L2 Unified 256 KiB (x24)
  L3 Unified 30720 KiB (x2)
---------------------------------------------------------------
Benchmark                     Time             CPU   Iterations
---------------------------------------------------------------
(LLVM, before)
Triangle/Hello             1.02 ms        0.500 ms         1000
Triangle/Multisample       19.3 ms        0.984 ms         1000

(LLVM, after)
Triangle/Hello            0.845 ms        0.439 ms         1673
Triangle/Multisample       6.95 ms        0.781 ms         1000

(Subzero, before)
Triangle/Hello             1.15 ms        0.516 ms         1120
Triangle/Multisample       40.3 ms        0.469 ms          100

(Subzero, after)
Triangle/Hello             1.19 ms        0.474 ms         1120
Triangle/Multisample       11.8 ms        0.920 ms          747

Bug: b/147802090
Change-Id: I15729552f01a509a5cfce20cd7de06d0b764cf0a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/47969
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>