cmd/compile: miscompilation of some arithmetic and conditionals on arm [1.15 backport]

[gopherbot]

@zx2c4 requested issue #42876 to be considered for backport to the next 1.15 minor release.

@gopherbot please backport this because compiler bugs in bitmath seem worrisome

[toothrot]

Approving as a serious issue with no workaround.

[gopherbot]

Change https://golang.org/cl/282432 mentions this issue: [release-branch.go1.15] cmd/compile: do not assume TST and TEQ set V on arm

[gopherbot]

Closed by merging d9b4ef5 to release-branch.go1.15.