Granular @fastmath flags

[mikmoore]

@fastmath is currently an all-or-nothing construct: one opts into every available flag (which is subject to change with new LLVM versions) by using this macro. There is no syntax to invoke a specific subset of the available flags.

A syntax for opt-in flags like @fastmath (;reassoc=true,contract=true,nsz=true) EXPR would be welcome for a more controlled @fastmath experience.

[giordano]

For the record, tangentially related to #49387 and #49405 (comment)

[longemen3000]

maybe something like the @assume_effects macro?

@fastmath :contract :reassoc Expr #for applying those flags to the expr
@fastmath Expr #default behaviour, all flags on