cmd/compile: binary.BigEndian.UintXX should compile to a load+bswap on amd64

[rasky]

Given the existing pattern-matching for binary.LittleEndian.UintXX functions, the functionality could be extended to support also binary.BigEndian, by doing a straight load followed by a BSWAP instruction.

[bradfitz]

/cc @randall77

[dsnet]

Possible duplicate of #14366.

[dsnet]

Related to #16832

[gopherbot]

CL https://golang.org/cl/34636 mentions this issue.

[gopherbot]

CL https://golang.org/cl/34635 mentions this issue.