cmd/asm: add 'insert program mask' instruction for s390x
This CL adds the 'insert program mask' (IPM) instruction to s390x.
IPM stores the current program mask (which contains the condition
code) into a general purpose register.
This instruction will be useful when implementing intrinsics for
the arithmetic functions in the math/bits package. We can also
potentially use it to convert some condition codes into bool
values.
The condition code can be saved and restored using an instruction
sequence such as:
IPM R4 // save condition code to R4
...
TMLH R4, $0x3000 // restore condition code from R4
We can also use IPM to save the carry bit to a register using an
instruction sequence such as:
IPM R4 // save condition code to R4
RISBLGZ $31, $31, $3, R4, R4 // isolate carry bit in R4