Use ASSUME in op.c:newBINOP

This causes op.o to drop by 56 bytes for me under clang.
[perl #119973] Treat initial { in format args as block

Commit 705fe0e5f8a inadvertently caused the hash-vs-block disambigua-
tion to apply at the beginning of a format argument line.  This commit
restores the implicit ‘do’, but only when the opening brace is the
first token on that line, not embedded within an expression.

In other words, this now has a block as before:

format =
{ foo => "bar" }
.

but this still produces a hash (in 5.16 it was a block):

format =
+{ foo => "bar }
.