ARM64: Optimize pair of "str reg, [fp]" to stp

[kunalspathak]

str     x2, [fp,#16]
str     x3, [fp,#24]

can be combined into stp if the stores are happening in subsequent memory.

stp x2, x3, [fp, #16]

I collected no. of such str pairs in framework libraries and found approx. 33K pairs in 16K methods.

Details:

str_str_fp_to_stp.txt

category:cq
theme:optimization
skill-level:intermediate
cost:small
impact:medium

[BruceForstall]

Related: #35133, #35130